I'm not sure why they shouldn't use that CPU? It is low power, good enough performance and functionality for a NAS. Sure, the Goldmont is from 2017 and J4125 was released the end of 2019 but what else could Synology use, Tremont are still not available for these kind of systems.
But I think...
Not an ARM fanboy, just not impressed with the x86 development for the last 10 years.
Well, maybe not 5 years, but it was almost 5 years before Ice Lake got here after Sky Lake and on desktop and server it still are Sky Lake with very small addition. Zen played catchup to Sky Lake until Zen 3...
X86 performance doesn't increase as it used to for the last 10 years and that have allowed ARM based devices to catch up. Apple have already shown how their mobile CPU's have X86 performance with much lower energi required, everybody try to replicate this now for the increasing data center...
Target is most likely small footprint servers for AI, think SuperMicro Micro Cloud servers, you could get ~180 blades in a rack cabinet. Another usage would be appliances where you want to add AI, like firewalls and NAS.
PCIe 4 is rather new still, takes time to develop and most current...
I think this will hit Intel worse then AMD. Rumors are that Intels next Xeon Ice Lake will have up to 38 cores and Cooper Lake up to 48 cores, so for a Intel system with 2*38=72 cores you need to buy 4 socket licenses while you get up to 128 cores with same licenses with a AMD system.