Where did this idea that full marks gets you banned come from
All the sources I can find are from 3 years ago. Since then no one has supposedly been banned for playing full marks
It isn't full marks itself that gets you banned, but rather the fact that JC will manually check the accounts of people in the group if anyone gets reported. The reason JC will give for the ban is that "you are using a modded client", not "you are cheating in a full marks". So while full marks isn't the direct reason for a ban, it is responsible for you're account getting checked in the first place.
The mod itself is undetectable, in theory. For example, if you play the game with the mod installed you could play for years and are still very unlikely to ever get banned. However a lot of people with relatively new accounts who are using the modded apk are constantly posting here on this forum that they have recently been banned: Usually these post coincide with a full marks or a few days after a few marks has finished.