It's an insult because people who act like maximalists rarely call themselves that. It's other people that do it like it's something bad.
I'm probably a maximalist because I see Bitcoin as something superior to existing altcoins. Maybe one day Bitcoin will have to be upgraded and we'll move to a newer version but for now there's barely anything comparable. Look at top altcoins. XRP? BCH, BSV?

TBH most of the Bitcoin maximalists I've encountered do refer to themselves as Bitcoin Maximalists or Maxis.
Others go as far as defining themselves as Bitcoin 'Toxic' Maximalists!

...
BCH is not Bitcoin.
BSV is not Bitcoin. It's even a fork of BCH. Its in the license agreement ...
-
https://github.com/bitcoin-sv/bitcoin-sv/blob/master/LICENSE"The Bitcoin SV blockchains are defined,
for purposes of this license, as the Bitcoin blockchain containing block height #556767
with the hash "000000000000000001d956714215d96ffc00e0afda4cd0a96c96f8d802b1662b" ..."It's a total nullity based on a sham.

XRP ... no comment ... ROTFLSHMSFOAIDMT