Why was the PS5 the target?
Just after the bit suggested in the OP, they talking about how they got their first PS5 devkits in August 2019 and started looking to see how the game ran on it. And at that point they were noticing points where it was crashing back to 30fps from 60, which showed them where there were stalls and bottlenecks that needed to be fixed.
Remember the key thing is that all these changes were being done by the technical design team, so it wasn't impacting the engine at all, it was all the event scripts and blueprints running on top that were causing issues. Basically thats a shitty thing to deal with because its a step removed from the core code in terms of who's responsibility it is. The designers are tasked with implementing events and debugging the "action", but performance issues (beyond obvious fuckups like spawning too many entities on a certain trigger) aren't typically their wheel-house.
Fortunately it seems like their technical designers had solid coding skills so were able to work back and fix stuff from their end, but that's not always the case, and when that happens it can result in a lot of discussion and finger-pointing as to who's exactly at fault!