The argument is not that the advice is aggressive in itself, but that it should have been accompanied by a warning about the potential bad experiences in contributing to OSS, much like one might warn that a beach we're recommending can have particularly strong currents.

EDIT: actually I'm wrong; the argument is both that it should have been accompanied by a warning, and that not doing so (and stating it in a generalized way) ignores the experience of people who already had a bad experience contributing to OSS.