The last I heard (but I might’ve missed further conversations) we weren’t going to include appstream data inside the snap.yaml, but in a separate file.
In any case, an appstream id can refer to the snap itself (I think snapcraft calls it a bundle), or the apps themselves (and I think the latter is more common). For the former, id at the toplevel is correct, but for the former, you really want it to be in a map, i.e.
appstream:
id: the.id.of.the.snap.itself
# and more appstream data of the snap itself
apps:
foo:
id: com.example.foo
# and more appstream data of the foo app