Many have interpreted the OP's classification of their own comment as 'mean' to equate to abuse or bullying, and so feel condemnation is appropriate based on the limited information shared.
Comments like the above, and referring to other posters here as pompous puts you a long way off the moral high ground.
I read the first page or two of this thread a few days ago and only returned to it again today, and I'm finding it all fascinating!
I'm not sure if I'd go so far as to describe it as pomposity but I definitely have reservations about the level of presumption and dismissiveness pervading the thread.
People seem to be starting from a position of presuming the OP is unreasonable and/or irrational, rather than reasonable and/or rational. I find that interesting.
Then there seems to be a general presumption from most contributors that the mean comment was something that constituted "abuse" and that therefore the OP was fair game for the response of the unnamed celebrity. When I read the OP what I imagined was along the lines of:
[celeb]: "Look at these homemade scones I made while on lockdown!"
[OP]: "They look about as wooden and unpalatable as your performance in XXXX!" (Hypothetically the celeb is an actor!)
Something a bit pointed or sharp, mean
spirited rather than actually abusive...
Needless to say, nobody knows where precisely on the rather subjective spectrum of meanness / abusiveness the OP's comment was, because the OP hasn't given any further information, and for good reason. But I don't see why them referring to it as "mean" should equate it to abuse or bullying.
If one presumes the OP is a reasonable and rational person then one has to wonder at what the defamatory response must have been, to engender such a desire for rectification on the part of the OP. But it appears that the majority here (starting from a presumption of the OP being unreasonable) don't seem willing / able to recognise that a defamatory tweet that is likely to have been read by more people than read most local / regional newspapers, could be quite harmful to a person.
Since the OP's employer has acknowledged that they had no business engaging with him on the matter, I don't see why he can't / shouldn't engage a solicitor to write on his behalf to the person in question asking for the offending tweet to be remedied in whatever way might be best. But then I'm making presumptions too - that the OP is a reasonable person and that when they describe something as a wildly inaccurate claim portraying them in a very poor light, then that is a fair description.
An intriguing thread all round...!