There is one misconception there. Zen3 released on PC, a month before the PS5.
That means there was no time to make it into the PS5 SoC, unless Sony was willing to delay the PS5 release date by many months.
Just consider that AMD always releases the APUs around a year after the CPUs with the same arch.
At that time, the most up to date APUs from AMD, ready for production were Zen2. Zen3 APUs were far from being ready.
A similar thing might happen with the PS6. If it releases at the same time frame as Zen6, then it will probably use Zen5 APU.