Yes, you are right, and I remember BKK was served from TPE and maybe MNL. But it's been so long I could easily confuse NW's and UA's beyond TPE destinations.

The US and Taiwan have had an open skies agreement since 1998, required for 5th freedom rights. The wrinkle is UA also has a JV agreement with NH that could carry a commitment to coordinate sales and service to certain markets in Japan and Asia. The details are not public, so we have no idea what that might preclude UA from doing on their own from TPE.

Regardless, I would be shocked to see beyond TPE flights, more than GOH shocked me. BR, CI and JX already fly everywhere. UA does OK flying to/from US territories and places not served or little served by other major carriers.