Eugen Rochko is CEO of Mastodon gGmbH which is the company which develops the Mastodon software. It isn’t the software itself; that’s open source and could be forked and developed by anyone. Nor is it any of the instances which can be operated by anyone who wants to and they don’t even have to use Mastodon’s code to do so.

I could set up a company to run an instance and call myself CEO if I wanted, it would only mean something to users on my instance, I guess.

The overlapping names are, probably unintentionally, slightly confusing.