Being a free speech advocate when it benefits your own views is an oxymoron. Defending free speech by definition means defending people's right to say things you disagree with. If you only defend people's right to agree with you then you're not a free speech advocate.

I think that's precisely what they're saying: a lot of people talk about "free speech" because free speech benefits them. When you're in a dominant position, free speech doesn't threaten you. Or at least, you think it doesn't.

Few free speech advocates think you should be free to call for violence. That's free speech that does threaten them. Speech that calls for discrimination against groups they don't belong to, however, is perfectly fine -- they know that they're too important to be threatened by it.

So practically all self-described free speech advocates do put limits on free speech, and there aren't any True Scotsmen.

No. The point being made is that if your speech is being suppressed for whatever reason, you will care about free speech.

If you're not being suppressed, you won't even realise you have it, and will happily advocate for censorship.

That's a very cynical take. You don't need to have your own speech rights threatened to care about free speech, and many, many people are capable of defending free speech even if their own views aren't being "suppresed".

My political opinions are reasonably mainstream, but I'll happily defend the free speech rights of groups I disagree with (e.g. pro-lifers.)

If you don't support someone saying something you find disgusting / horrifying / etc, you don't actually support free speech.

'Free speech for my side, but suppress the other side' is not free speech. This is what Facebook / Twitter is engaging in.

You didn't see ANY suppression from them when Trump's tax returns were leaked like they're attempting to do with Hunter's emails.