Its not an ad hominem attack. Saying Stallman's arguments are wrong (or, in a weaker form, that they should be dismissed) because of the tactics similar to those of the LaRouche crowd similarity would be ad hominem.

Saying that Stallman's presentation of his ideas is, in fact, alienating to people who aren't already converts, and that the presentation's similarity to LaRouche extremists tactics is helpful in understanding why that happens (whether or not it should happen) is tactical criticism.

There may be a problem with it, but it isn't even remotely ad hominem.