I think the microaggression is not explicitly stating that those negative experiences exist while making that broad statement ("contributing to OSS is a good way to get started"), and thereby disregarding/minimizing/ignoring the people who had those experiences, harming them.

Oy. An article or talk where every point includes a disclaimer (or even a YMMV) would be pretty tedious. Katie started her talk saying it was an anecdote and it's too bad that a blanket statement in the preamble isn't good enough.

Oh, I don't disagree, just passing on what I understood from the tweets :)