There's a large number of e-gaming companies sponsoring OSS as a means of CSR and advertising. For smaller OSS, it is the majority of corporate donations as other types of companies don't bother. It's not likely that Mastodon encouraged those donations, but other companies aren't likely to reach out.

Doing further research, Mastodon's sold out on those tiers of sponsors[0], so they're likely in the lucky position that they could pick and choose.

[0] https://sponsor.joinmastodon.org/