Of course it's nonsense. Nobody says otherwise that is being serious. Google greatest quarterback of all time and tell me how many of the lists say Mahomes and how many don't say Brady. Honestly, the only place you hear a discussion about Mahomes being GOAT is here on this message board. I have never seen that debated on my Chiefs board. I wouldn't be surprised if someone on Chiefs Planet said something like that, because they seem to love trolling other teams. But I'm sure they know otherwise. I don't go there because it's a cesspool.
I think if his career stays on this track and he plays to a normal age, he will be considered with Montana/Manning/Elway types. If he somehow goes on a heater and wins 4 or 5 more, I'm sure there will be debates as there are with Jordan, Kobe and Lebron. He is doing things that have rarely been done. But it's nonsense to have that talk now.