It’s been a while since I’ve played with the Spotify custom tile, but from the top of that thread:
I wonder if you had a tile that started some sort of playback on an ‘inactive’ device through a smart home integration (eg. running a rule), if you could then select it as the target from the custom tile (using the speaker icon - see the video from the original thread)
The Custom Tile is completely standalone though as it communicates directly with the Spotify API. It’s open source, so in theory it could be extended and modified.