It made me laugh at myself, for I should have known better.

I had often been in England and could have told anybody that you mustn't too abruptly or obviously refer to what the other fellow does, still less to what you do yourself.

"It isn't done." It's a sort of indecent exposure.

It's one of the invasions of the right to privacy.
In America, not everywhere but in many places, a man upon entering a club and seeing a friend across the room, will not hesitate to call out to him, "Hullo, Jack!" or "Hullo, George!" or whatever.

In England "it isn't done." The greeting would be conveyed by a short nod or a glance.
To call out a man's name across a room full of people, some of whom may be total strangers, invades his privacy and theirs.