Yep, you’re completely right about remote build and Launchpad lacking proper support of platforms.
We have an issue in Snapcraft here (New remote builder fails with some platforms definitions · Issue #4858 · canonical/snapcraft · GitHub), outstanding work in our new craft-platforms library here (Implement `get_platforms_snap_build_plan()` · Issue #5 · canonical/craft-platforms · GitHub) for Launchpad to use, and a mention of the limitation in the documentation here.