it wouldn't be as they still held on to the "we believe in generations" statement
This is wrong.
In the 'we believe in generations' Jim Ryan never said nothing about not making crossgen games. He said the opposite: that they were going to keep supporting PS4 during several years because they had over 100M MAU and they were aware that (even if breaking all gaming history records for a console launch) only a smal portion of them would be able to jump to next gen during the first few years.
He also specified in that interview that with "we believe in generations" he meant that they wanted all PS5 games to use the new features they added compared to PS4 (haptic feedback, adaptative triggers, 3D audio, much faster loadings, etc). And this is true even for crossgen games.
Not to mention Santa Monica had never developed a cross-gen GoW before
With the current engines and current hardware is much, much easier to make a crossgen game. Next gen consoles use the same type of hardware than in the previous gen but way more powerful.
In previous generations the change was way bigger, so basically devs had to remake/remaster the game almost from scratch to have it both consoles.
Thanks to PSN and XBL MAU they know the amount of active players that didn't jump during the first years, so they now there's a lot of market there and pretty likely this is why they designed this gen's hardware to be more similar to the previous one (also to reduce the changes needed in terms of engines and tools to migrate to next gen).
This is why a ton of studios -not only Santa Monica or Sony- is making crossgen games for the first time.
Ratchet and Clank of all things was gonna be next gen only while Ragnarok wouldn't be? Not many people I imagine.
GoW 2018 sold 20 million copies and Ratchet games sold way under 5M looking at the numbers we have (Rift Apart sold 1.1M on the first month aprox.). So if they had to sacrifice around half of the sales of one of them, it was better so sacrifice Ratchet sales.
Same goes if you compare GT7 vs Destruction AllStars or Horizon 2 vs Returnal.
They know next gen only games during the launch window will sell a limited amount of copies, so now that their big ass AAA games cost some hundreds of millions of dollars combining development and marketing they need to play it safe and make it available to the biggest amount of players possible to make sure the game becomes profitable.
For the same reason they port the game to PC too, even if some years after the console release.
It's the best option for them if they don't want to turn their games in to a lootbox microtransaction fest.