It's not like improper use of "steal" is unheard of, I see all the time people use "I'm gonna steal that" and similar even when it applies to things openly given for free. And considering that it's quite clear that the MIT allows others to take without sharing back (it's the main difference with GPL) I'm quite sure the commenter was aware that it wasn't really theft, yet chose that word probably with the intention to insult the practice, rather than as a fair descriptor.

So yes, you're right, it isn't theft... but I don't think that was the point of the comment.