According to their FY2021 990 filing, they had $442k of "other" expenses.
It seems like they're required to disclose what those expenses are on the Schedule O form if they exceed 10% of all expenses, which they do, but I don't see the expenses enumerated there. (edit: oops, now I see)
I am, err, not a tax professional so I stopped here..
Out of their $443,020 expenses in 2021, the biggest line item expense was $187,262 for "CONSULTANTS." Their next biggest was $169,383 of merchant processing fees. It will be interesting to see their Form 990 for 2022, which is presumably the "last year" Ben Fry is talking about..
developers, especially developers that work with a large contributor base on open-source projects, often get paid by non-profits under a consultant label