Any master against me can call mate in 40 and be right.
I did remove grandmaster though. Grandmaster is a lifetime title, while master is something earned and taken away. Thus former great can earn grandmaster and then go senile and play at a level below me - well at least in theory, I don't know if any exist but I'd love to play one just so I can say I beat a grandmaster.
I doubt any grandmaster will have any problem in forcing mate in less than 40 against me. Many real games don't get beyond 40 against opponents who are not nearly at that level.
You are still ignoring what multiple people have tried to explain to you.
"Mate in X" is not a show of bravado about how quickly a grandmaster things he can beat _you_ specifically. It's a mathematical property about a position. It doesn't matter if Carlsen or a chimp are sitting on the opposite side of the board, "Mate in X" means the same thing.