I disagree. I'm definitely not kidding myself, not because I'm 'nice' I don't think I am particularly, but because I'm self aware of what I type and don't.
I also know what comes out of my mouth, when and where, so I don't find it a case of thinking I moderate myself. I actively choose to moderate myself and my language on social media.
In real life I'm intentionally polite to shop assistants, call center folk, traffic wardens, etc even if they aren't, because I see what they get subjected to, and mainly (but not always) bite my tongue when subjected to road rage, because tempting as it is, retaliation generally doesn't make for a better situation.
However I'm in a wheelchair and when I get nastiness and deliberate ableism in real life, I'm liable to let rip back loudly and will match or take beyond, whatever level I'm being insulted or sneered at. It's my outlet, those people have IMO 'asked' to see what the person they're insulting thinks about it.
On here I mainly either ignore it, or point out that my PIP is what allows me to work and pay tax, and I don't have a motabillity vehicle, but am fine with my taxes going towards supporting those that do including to have a better vehicle than me, and can't work or pay tax.
I have once mimicked the user name of someone who bitchplops on specific threads, into an apt description of their behaviors, which has been the height of my unpleasantness on SM, and was a rare thing from me, but I'd very happily use those words in to them in real life and ask them what motivates them to behave so nastily.
I can think of other long term MN posters who keep their replies decent.