Uh no, the copyright, on the code at least, is not in the hands of a for-profit. It's in the hands of Eugen Rochko and several hundred other individuals.
Quoth the README: “Copyright (c) 2016-2026 Eugen Rochko & other Mastodon contributors” (and there is a link to a list of said contributors)
There does not appear to be a CLA or required copyright assignment.
https://github.com/mastodon/.github/blob/9fbf5954df1918d6190b38469e52426df20294c5/CONTRIBUTING.md
https://github.com/mastodon/mastodon/blob/07f91058a096bcfb92797e145bb7183ac5c6168f/CONTRIBUTING.md