An extremist can be one who is extreme in action rather than ideology

Yes but they need not be.
Of course a person with extreme views is an extremist whether or not their behaviour is extreme.

A moderate person who posts mainstream views on a forum would be seen by extremists as posting twaddle to the chagrin of the extremists on the forum.

The anomaly here is that the extremists dominate certain threads to the chagrin of normal decent mainstream people who find their views marginalised.