I think the problem here will be that Zen 4 would be far slower on DDR4 due to the way it works. Like, if the uplift is because of the IF speed and DDR5 is capable of reaching waaay higher clocks then DDR4 could hamper it.
I could be wrong, but that may be why they are adopting this stance.
DDR5 is still kinda new, and all DDR is stupid money when it's still new. That said many put DDR5 on Alderlake, which IMO was a silly move. It really doesn't make a huge difference, and AL isn't significantly faster than Zen 5000 series, so yeah kinda a waste of money.
If it makes the difference here? it may be worth it. You gotta remember we are talking about the bleeding edge here. I would hazard a guess and say they will still be knocking out 5000 series chips for quite a while to come.