Um, you find EPYC working wondrous? EPYC is Zen.
Zen is an x86-based core processing architecture, not a complete CPU. Equipped with necessary subset of bus controllers, it does multi-CPU in EPYC (non-P variants).
Desktop and mobile versions (Ryzen) have this subset cut initially or just disabled of course, some EPYC's (P variants) have this subset either cut, or non-functioning and disabled or just disabled.
I don't remember exactly about HEDT (Threadripper), but as it's EPYC socket and I/O die based, it probably should be able to support multi-CPU usage.