It seems that all the device manufacturers (they mention Apple, Samsung, and Google) are using BeiDou anyway. I don't know if that includes Garmin watches, though.
This is only to make getting a fix faster - A-GPS downloads the almanac data via the internet (fast, but needs internet) instead of directly from the GPS satellites (slow but works everywhere). It doesn't make it any more accurate.
https://www.reuters.com/technology/us-agency-probes-risks-fo...