What he would say to his mates is of absolutely no relevance. I can call my mates from a pig to a dog and them do the same to me and it means nothing. I am talking about whether he would call a stranger a jabbering lunatic in person and if not, which I would hope would be the case, then its not acceptable to do it online. But you seem to be defending him for some reason.
As I said, I wouldn't use those words myself but I agree with the sentiment of saying that what S.M. claimed is daft. Anyway, as he told me to f.ck off and called me a patronising t.at, which I very much doubt he would say to my face, I'm not all that inclined to feel that upset about it.
It is perfectly acceptable to make such statements when someone makes such a ridiculous statement as saying that all coaches from outside of the UK think goalkeepers don't matter. That's just opening yourself up to ridicule and letting everyone know that you are a brain donor.