That's kind of missing the point. You can object to someone's actions even if they say their actions are for a righteous cause. Say someone says they are anti-PETA, and the response is that if they object to the ethical treatment of animals, that must mean they support animal abuse. It's a disingenuous argument because what the first person meant was possibly not that they object to the literal meaning of the name of the organization, but instead that they have an issue with things people calling themselves members have been doing and saying, and it's being deliberately misinterpreted.

Now maybe that actually was what they meant, or they meant both; there are people who support animal abuse. Maybe they are wrong to be anti-PETA, for reasons other than the name. But that type of argument is still a dishonest smear aimed at unconditionally rejecting all possible criticism.

They're not saying fascism isn't bad, they're saying that any group can call themselves whatever they want; the name itself means literally nothing, and so shouldn't be the foundation of any argument about why opposing it is bad.

In other words, 'they're fascist because they're against the group that calls themselves anti-fascist' is a moronic argument, and should be replaced with the more direct, and more accurate, 'they're fascist because they do XYZ fascist acts'. Forget the name, focus on the acts.

After all, you understand that opposing someone who self-labels as "pro-life" doesn't make you "anti-life", right?

The label could be accurate, or could be inaccurate, but the bottom line is that the name does not define the group, and every group that names itself is going to choose a name that makes them look/sound like the good guys—both benevolent and malevolent groups.