You're confusing the fact that Protestants were in error (example, Sola Scriptura) with the fact that they have beliefs.
Of course any other fundamentalist besides Catholics are going to be in error.
But the fact that we take our religion seriously, believe it's true for EVERYONE (whether they accept it or not), are anxious to spread it, are militant about it, bring it into our daily lives -- those are traits we share with any other "fundamentalist" group.
The fact that Protestants are ignorant about this or that has nothing to do with the term Fundamentalist.
Long story short, "fundamentalist protestant" just means "real protestant" or "protestant who actually takes his religion seriously". But no one ever said "real protestants" weren't in error.
So-called "Mainstream Protestants" (the only other type, besides "fundamentalist" protestants) are just like any other non-Christian, and so they are a joke. I don't even consider them to have a religion.
The only protestant worthy of the name is a fundamentalist protestant. Likewise, the only Muslim worthy of the name is a fundamentalist Muslim. If they look/act/dress/marry/live like those outside of their religion, how can they identify with that religion?