Plus Fog Creek is more on an anti-start-up since it explicitly did not want to grow fast at all costs. That's why Trello was spun off.
"Startup" is a nebulous term with no firm definition, so these kinds of arguments are bound to happen. Some people still say Facebook is a "startup" because it has the fast-paced environment with the normal superficial cultural trappings of a tech startup (food perks, no dress code, casual interpersonal culture, etc. etc.).
Ultimately it's probably just a dumb semantic argument that's best avoided.