Look here: https://www.google.com/search?q=antifa+logo&tbm=isch&ved=2ah...
The google image search for "Antifa logo" does not show the inverted triangle, except in a post explaining the ad we're all talking about.
Nobody thinks of an inverted triangle when thinking about Antifa. Some people in the antifa movement may have used the symbol, but it's not associated with them enough for an ad calling them out to use it.
Why run an anti-antifa ad and use a symbol that people don't associate with antifa?