If they had made a cash donation, it would have been an unambiguously good thing.

Equity, however, creates a conflict of interest. If the non-profit Postgres organizations make money when Citus makes money, then they have an incentive to favor Citus over other companies.

Let's say there's a Citus competitor. The competitor implements a feature a bit differently than Citus. Does Postgres support both organizations equally? Or if there's a technical decision to be made, and it involves making one company's code incompatible with base Postgres, who do you think will win?

It's a free market, and the parties involved are perfectly free to make this arrangement, but it generates potential problems.