I'll just risk it and say you have not the slightest clue about that 48h embargo.

My guess is that 90% of the people watching the show were impressed by "All those brand new XBox only games". It doesn't require saying "exclusive" here, it's the impression that counts. If you impose a shitty 48hours embargo for the producers of all the multiplats, that shows just how insecure the company is currently.

My thoughts:

Without the Starfield direct, I would say the Xbox Showcase was slightly better than the PlayStation Showcase, despite their much more substantial first party presence. A lot of their first party was updates from previously announced games. Felt a little down on seeing the Avowed gameplay reveal and I was hoping to see a better vertical slice of Hellblade 2. Fable had a bit of gameplay but it was mostly an in engine trailer. Updates for SoT, FO76, and ESO were some of the low points for me, even though these weren't bad showings by any measure. South of Midnight and Clockwork Revolution, despite not being CGI trailers, felt the same as seeing one since those projects are far off. I will say Clockwork Revolution intrigues me with its setting. As for third party, I was surprised with some of the reveals, with new IP's from Capcom and Atlus, although the latter felt a bit underwhelming. I can't remember the last time an Xbox Showcase had a reveal as big as Star Wars Outlaws. It feels like these massive third party games have been missing from Xbox Showcases since the beginning of this generation.

Xbox Showcase w/out Starfield: 6.5 out of 10.

Honestly, not a lot from Microsoft's first party really impressed me from a technical level, but Starfield blew me away completely. Everything about Starfield felt ambitious. Bethesda is on another level when it comes to intertwining RPG elements with personalized builds, skill trees, NPC's, exploration and overall setting. I didn't enjoy the base building in Fallout 4 too much, but I'm feeling a lot more excited about the ship building, crews, and outposts in Starfield. I feel like the VATS mechanic carried FO4's core gameplay loop, but it looks like Starfield has better gunplay, and the ability to fly compensates some of my concern, although I guess VATS could make a return in some form. No 60FPS option is a bit of a disappointment, but to be honest, I don't play Bethesda games for their gameplay, and 30FPS suits Starfield's game design. This is the best deep dive I've ever seen for any game, but I don't think the majority of games have the content and systems like Starfield, to be able to pull it off. This feels like its going to be a GOTY/GOTG contender, and Microsoft's first major tentpole title in nearly a decade.

Xbox Showcase + Starfield Direct: 8 out of 10.

This amount of pickiness tells way more about yourself than anything company related seriously.

Xbox had a main show with limited time and they want to maximize the impact of every mins of it. This kind of embargo is logical in this regard, if Xbox lends 3rd parties precious time of their shows that have a higher audience than any of those 3rd parties could lend by making their own show it's only logical that they would want to maximize the benefits of such for their platform.

From what I've read, Microsoft does these embargoes because PlayStation's social media channels absolutely dwarfs them, and they wouldn't get nearly as many views on their own channel if Sony could just upload a trailer a few hours after. It makes a lot more sense as to why Microsoft would embargo some of these announcements despite them being multiplatform. 

Having a 2 day marketing deal for showing their game is insecurity? Bro, you need to chill. As for the first part PS doesn't list multiplats at their shows either... so yeah we've grown accustomed to drawing out info based on what is said.

Sony /  PlayStation /  PlayStation's social media channels are unaffected by those supposed embargoes as they are presented online. Those only affect actual third parties that have shown content during the main show. Nothing prevents Sony from posting the very same trailer shown and adding 'also coming to PlayStations' and reaping the exact same benefits with/without embargoes. So really doubt your sources' conclusion on this.

It only prevents said thirds party from marketing the titles themselves for other platforms or for coordinating with other entities to market said material on other platforms for 2 days. This is only logical when considering this kind of exposure for 3rd parties is a huge opportunity and this embargo only ensures that there's reciprocity with said opportunity in the vicinity of the show itself.

Then add 6 points for effective marketing and for causing fake outrage. 

It was an editor from Push Square who theorized it. 

Also, its not about PlayStation media channels being unaffected, its about Xbox media channels getting overshadowed and losing views, due to the algorithms that favor PlayStation since it is a much more popular brand. 

@edit ok, I now see what you mean. And yes but it's kind of logical and should be somewhat expected as it protects the mutual benefits of such exposure for both the third parties benefitting from it and the one providing it.

Besides what those stories don't mention his:

A) Is it strictly an Xbox thing?

B) If it is, are there other forms of comparable restriction/payback from exposure in competitors' shows?

Because the way I see it, if a 2 days embargo is the only thing asked from 3rd parties to be provided with such exposure it is a very generous offering from MS.