AMD ships their engineering samples at very low clocks; I am pretty sure the method folks are using to overclock them is intended for clock tuning 'in the field' as they qualify their parts. The ZS is a newer stepping (and likely a newer variant of TSMC 7 nm) with better voltage scaling and more clock headroom. Even so, they are worse than the release parts - my ZS1406 can just about match a 7702 before the limiters start kicking in, whereas the 7742 clocks a bit higher and Threadripper lives in a world of its own.
I'm also inclined to believe the limiter is not entirely VRM-driven; if I disable HT, I can hit 3.2 GHz on all 64 cores in Cinebench and just about match 2.5 GHz + HT, but the chip draws horrifying amounts of power. It's an interesting tradeoff, if I had water on the VRM's I'd run at 3.2 GHz and get my good single thread performance without losing multithreaded performance, but with just air I don't think the VRM's can sustain it.