We don't really know to be honest. I mean could that specific UE5 demo have run on Series X as well? I would've thought so, but after reading this quote from a MS exec, I'm starting to have my doubts. All we know for certain is that Sony has shown Spider man with zero loading times and we've seen State of Decay loading in 10 seconds. I also think Sony wouldn't have put so much effort in their SSD tech if it would just mean a bit less pop-in and load a couple of seconds faster. They practically build their whole console around it and are convinced it's the key to the next generation, which a lot of developers seem to be agreeing with.
It wouldn't surprise me if MS releases a mid-gen Series XXL with a faster SSD.
SoD2 loaded the entire game in 7 seconds with zero optimization done to increase the SSD benefit, it was simply the result of installing the game on the XSX SSD. Spider-Man was a tech demo of the game with no character. Not really an apple to apple comparison and not surprisingly you're incorrect on the time it took the XSX to load the game.
The SSD isn't just there for faster loading times, it's supposed to change core level design as we know it, get rid off elevator rides and make game development much easier. A MS exec has now literally said developers need to use "tricks" to overcome the gap in throughput on Series X. It simply means either both versions will still have elevator rides, or they'll have to make changes to the core level design on Series X if ps5 is being targeted as the base platform. Maybe nobody will notice the difference, maybe they will. That we don't know yet.
Both consoles have it strengths and weaknesses in specific areas and it will be up to developers which platform's strengths they'll prioritize. Like always, that's a simple matter of which one has the biggest install base and can potentially sell the biggest numbers. This gen we're seeing multiplatform games outsell 3 or 4:1 on ps4, so there's that. But I guess we'll see.