I still don't quite understand your comment.
>If there are advantages to ATProto over ActivityPub in this kind of deployment setting, they don't seem clear enough to offset the weird corporate parentage
The advantage of atproto is that it's literally "the web". Every app can aggregate from entire network, there's no isolation by default. It reduces to the degenerate smallest case just the same — you can have a single-user app that simply displays content from your PDS. But you can also start aggregating things (e.g. comments left by other users, which are stored on their PDS's). The whole big idea of atproto is that every app is effectively a CMS for app-agnostic "model" web.
What is the concrete thing bothering you? AT is already in the IETF process. It's not some weird corporate thing.
A "lite" version of Mastodon that dispensed with all of the complexity involved with managing multiple accounts and was optimized for a single user might be nice but with a hosted account you're paying for data storage (which Mastodon seems horribly inefficient with) so in practice that extra complexity is moot.
The thing there is it destroys the objection people have about having to find and fit in with a particular Mastodon server community.
It also makes Mastodon look more like the original blogosphere and less like Twitter, which is a good thing.
It is just an entire Mastodon instance with one account, though.