I'm not sure I buy that. It's not exactly a non-restrictive phrase in this case because the sentence "I like how someone did so with poor grammar." does not seem like the original intent. You can shoehorn that argument I suppose, but it certainly just looks more like an errant comma.
If I were to guess, I think OP thought they had an adverbial phrase (I think that's the right term) that needed to be set apart with a comma, but, in actual fact, it both was not and did not.