I haven't hacked away at the bluesky api but isn't the aforementioned "app.bsky.feed.getPostThread" called against an instance of a bluesky relay hosted at api.

I haven't hacked away at the bluesky api but isn't the aforementioned "app.bsky.feed.getPostThread" called against an instance of a bluesky relay hosted at api.bsky.app, as opposed to a PDS or an appview? That being the case, when you want to get posts of a thread, the information of which posts belong to which thread are the responsibility of a relay, which is doing the firehose-level-aggregation of which posts belong to which threads, am I misunderstanding?