His Bio does include 'software engineer', and he's worked at Microsoft before from here. I get the sense that the problem that people have with Mastodon is that they've subbed the PRs, but they've not been approved. For example, there's apparently a 12k line PR for Groups that the main guy isn't keen on.
It can be tricky getting your own code into other people's projects - the maintainers need to have a lack of ego that can be difficult to achieve. Generally, developers react better to code, than to ideas about code, but you want to convince them to look at future code, and not waste your time creating it, by floating the idea first, so you can end up in an impossible situation. Subbing big PRs only for them to be dismissed by a maintainer is what causes people to rage-quit the project altogether (for Lemmy, see Tesseract UI, and the 'bulletintree' guy)