"It's not botting, it's AFK training"

It must either look very convincing, or it's really damn convincing. I don't think the GMs are *that* gullible...?

They are.
You have to remember that GMs, by and large, have no idea how to play the game. They don't know what things that happen on the screen require human action, and what happens by itself (pet buffs, etc).
I assume they have been trained to spot bots by their jittery movements and random teleports. While those are typical of people botting using certain 3rd-party "trainer" programs, they obviously don't happen with spin4days'ers (nor weight-on-a-key botters). So those don't get banned.

Edit: and as for people who have more "realistic" macros, the only way to spot them is to try to engage them in conversation. Maple is a very repetitive game, and when grinding for weeks on the same map one does develop a pattern and rhythm of attacks and buffs. The DS in Mazz's video never reacted to people talking about him, nor to being KS'ed by the lumi, which is quite suspicious. But. Korea is on NexonNA's tail for every possibly-legit NX-buying player they ban... which is one reason NexonNA prefers to just ban FM spambots.