Honestly think it has nothing to do with unified vs non-unified L3$ on the CPU. More likely explanations for DMC5 performance with framerates on PS5 vs Series X are:
1: PS5 was likely lead platform for DMC5 next-gen ports
2: Series systems have been affected with devkits being further behind due to transition to Gamecore (this probably even was the reason why Capcom originally said RT wouldn't be in the Series X version at launch and patched in post-launch instead)
Also, let's keep in mind that other Series X games like Gears 5 (Enhanced) and DiRT 5 are hitting 120 FPS with no problems so, even if PS5 CPU has unified L3$, that isn't the issue for the FPS dips on PS5 vs. Series X. Capcom has a bit of a history with sloppy performance of games on MS systems needing a later patch; RE3 Remake is a good recent example.
EDIT: I'd be interested if there are PC benchmarks of games like DMC5 (or other games with next-gen installments) running on Zen 2 CPUs, with 120 FPS options. Because I'm sure more than a few of them would have no issues in that setup, unless the bottleneck is the GPU.
Just seems a bit "jumping to conclusions" to peg the FPS performance in Performance Mode on Series X to unified vs. non-unified L3$; we'd see that type of problem regularly pop up on other PC gaming setups with ranges of CPUs (AMD, Intel) not holding steady 120 FPS. That hasn't happened, though.