You have to keep in mind that part of the reason for this is because Sony have generally been ahead in their API tools and devkits, and multiple 3P devs have alluded to their PS5 SDK essentially being a PS4 SDK supercharged with PS5 feature support added on top. It's not a complete retooling of devkit profiles and such the way Microsoft is doing things.
Thing is we don't know if they are asking IO behind the scenes; they very well could be. I think things like if PS5 could run native 4K if they tolerated frame drops can be answered by the community. Answer is obviously yes, but a caveat is that the drops likely would've been worst than Series X. Why? Because it's already running at 44% lower resolution to maintain locked 60.
Yes there is that one area where Series X drops into the 50s and a dip into the 40s when switching to sniper view (in that very specific flower field section), but that area represents less than a fraction of 1% of the game time experience. People can't suddenly use it as a means of implying that is a general drop Series X experiences for any significant amount of play time, since the actual time it happens is easily within a margin of error (MoE is usually something about 3% either which way).
OTOH, it's likely IO did try going with the game at native 4K on PS5 during dev and noticed the framerate dipped below 60 too often throughout the whole of the game to justify it. Maybe they also tried DRS but noticed the lower bound was too low for their own preference (I would guess maybe something to an effect of 1792p * 1008p, keeping the 44% amount factored in) and occurred too often, therefore going with a fixed resolution as a result.
That's just my own speculation; it would be nice to see spots like DF ponder more openly on that front to try rationalizing the results, and like I said maybe they are reaching out to IO on this behind the scenes. Maybe we will get a patch soon that bumps up the resolution for PS5. Maybe we will get one that addresses the small framerate dips in the Series X version. But some people take this stuff WAY too personally; the game is perfectly playable on both systems. Even the Series S version is perfectly fine for what it is.