Well just looking back as how things were between SNY and Epic around the Fortnite cross play dilemma, this looks to be a pretty big change in whatever relationship exists between them now.

It wouldn't be surprising, as MS was touting how great they were for getting ahead of things in terms of gaming advancement, by working with Epic on Fortnite cross play. We all know what that led to. The way I see it, it would be fair game for SNY to be using this as PR against the competition. A little Epic revenge perhaps? Tit for tat? Maybe SNY management hasn't played TLOU2 or doesn't get the point about revenge?

I'm mostly just joking about TLOU2 comment, and was mostly joking about the prior ring cost remark.

I feel that to the focus on UE is narrowing our thinking , it's been pointed out that Sony mainly uses in house game engines with great success on both a technical and sales level, so the main reasons a lot of studios use UE because of its familiarity in the industry and modularity doesn't apply and if they decide to use it they can just licence it like Sony Bend, so the more likely scenario is what has been stated, that is after working with Epic over a number of years on the PS5 SSD/IO and the positives that then flowed from that work with the UE5 showcase impressed Sony enough to take a small stake in Epic .

Now looking at the amount spent and combining that with both parties history with PS5's development , my conclusion is Sony was chuffed with the results of the partnership and its effect on the PS5's development so Sony looking ahead at future developments saw taking a small stake in Epic as a way of further cementing that relationship rather than it being a big financial strategy, being more of a symbolic gesture of were behind you going forward and so fencing off what they see as an asset in regards to future development, so it seems like it is a more general outlook than just UE/fortnite. 

 I feel we are in agreement,There are other benefits that's why I said it's a more general outlook than just UE /fortnite, but the likely catalyst seems to be the PS5 development and positive PR the two parts go together and cover the narrative position that you mentioned, plus the 250 million isn't really an impost since unless the bottom falls out of Epic's share price they should do all right.

Epic isn't publicly traded, so they don't have fluctuating "share price" like that, every investor has made particular deal.

Epic's CEO also emphasized the synergy in cross play, with Epic's platform used to allow "every platform" to achieve crossplay.
A good amount of Sony's issue with that was MS getting their fingers and eyes on Sony's customers, but Epic platform can avoid that,
with Epic's infrastructure also establishing vertical platform to address PC market from on own terms, not as 3rd party paying for distro.
I think a developing relation with Epic is also attractive for funding Sony's own games being ported to PC, since Epic can partially fund them
in proportion to expected PC sales, or for that matter Xbox sales if they go there too. Sony gets more total money from crossplatform sales,
while using their own capital more efficiently, allowing them to fund more game development if they wish. Of course they don't need
minority holding to do that, but it greases the wheels and keeps everybody on the same page if Sony has seat on Epic's board as seems likely.

Longer term the question is if there will be deeper integration of Playstation and Epic infrastructure, which IMHO is pretty likely at some level.
Integration of other media like film /TV also seems relevant, especially with Disney on board at Epic, furthering the appeal for both sides.

So many guesses, so I'll add mine to the list of conspiracy theories:

Cerny has been doing his thing for over a decade now. Chances are he spent time with Epic and went back and forward a few times. Epic has one of the most used third party engines and Sony needs third party support as always.

Over the last few years, between what Epuc needs and what the Ps5 needs there has been a meeting of the minds.

Maybe they knew the demo will light up the Internet and forums, maybe they didn't, maybe it was somewhere in between, but it's happened and the shit took off.

Between the UE5 use for third parties, and UE5 use for filming, Sony and Epic have things they could use from one another - hardware from Sony, software from Epic. Sony makes an investment.

I don't see it as anymore complicated than that. Maybe Sony was feeling things out, maybe epic was feeling things out. Maybe they both knew it might happen, maybe they both knew all along, but it's certainly been in the making a long time.

I doubt there is a big grand design more than getting better PR. A 1.4% stake isn't going to get you anything more than that.

250M (that could make 5 AAA exclusives) to have a 10min demo mentioning your console a couple times then some tweets are really big pricey.

Epic