Okay, without going into the actual beliefs of any of the parties involved,
Radical Feminism (as used by the comic) is a term used essentially solely within feminism circles.
Radical Feminism,(as used by society as a whole) is used much more widely.
Both of these terms can coexist 90% of the time, but when someone from outside of your grouping uses a term in a different way than within your grouping, they are not wrong, they use a word differently. The correct response is
'Sorry mate, but what do you mean by that'...'okay, well in my grouping we use it in this way, but now that we have established what we both think it means, we can move on' the incorrect response is 'No! You're an idiot who doesn't follow the rules me and my friends made up and never told you! You can never understand what we are talking about!'
This is not a feminism thing, this is a... I suppose dialect would be the best word to use for it, or maybe jargon. The same thing happens when military and civilians speak to each other, communists and capitalists, Collingwood fans and people who actually know who their parents are...
When a group spends time in insular discussion, its usage of language alters to better fit the requirements of the group. That is how language works. It is not a bad thing, but must be acknowledged in order to allow that group to effectively interact with other groups.
Looking at this comic, Erin knew exactly what the other person was trying to say, wilfully misinterpreted it to make herself seem more intelligent and claimed victory. Essentially, were I to make a comic demonstrating the concept I just described above, Erin would be the one wearing the straw man t-shirt.
Again, what either party believes is irrelevant. Using language effectively is essential for a discussion to take place