This is sorta good, very bad. In general, yes. Defining terms is super important for discussion.
It breaks down when people won't agree on terms or argue in bad faith, like this person is doing.
Hypothetical person A states that abortion is bad, and against their beliefs.
Hypothetical person B states that abortion is bad and they aid and abet efforts to stop access to birth control, put bounties on abortion providers/women who've had abortions, seek the death penalty for people involved with abortion, and to blow up medical centers/kill abortion providers.
They both have the same core belief, but person B is an extremist.
Now, let's say person C agrees with at least some if the least violent aspects of person B's goals. Of the ones listed, let's say they also believe abortion is wrong, seek to deny access to birth control, and are down with putting bounties on abortion providers/women who've had abortions. Do you think they will go "Oh, sure, a few of my beliefs are a bit extreme compared to the general population" or think they'd go "Me, extreme? No way. That's just you demonizing people who disagree with you!"
If one person isn't accepting definitions, it's hard to move a conversation forward.