"I really don't think they are, but are just pointing out errors, which is useful for the poster to realise and learn from. "
They aren't, they are being a twat.
My DD posts on FB whilst in work, sometimes as a stress relief, she works in MH. She has ADHD and Dyslexia, but she has mafde it to Management ad manages to write reports etc.
When she is writing on SM she doesn't want it to be an effort.
Until I had Brain Fog because of CFS, I didn't fully understand when she described concentrating on literacy as painful.
Her main FB 'friend' who often corrects her and posts lots of jokes about how she hands out a spelling test before going on a date, has never been able to hold down a job, despite having a BA/MA. It took her a lot less to gain a MA than it did my DD her lesser qualifications, whilst working full time from the age of 17.
By contrast my other DD doesn't post on FB often because she spends a while making sure everything is correct, so now doesn't bother. Luckily she has a online friendship circle who don't care about Spelling/Grammar.
Pointing it out can potentially isolate someone further. People with various conditions fight for acceptance and struggle to make friendship groups they way mainstream people do.
It can often be a case of knocking someone when they are already down.
But people who are lacking in someway always need something to feel superior about, so they pick easy targets.