Well he has a point with the time we know it took for gohan to turn mystic. This is true. However there's a lot of gaps where there is no set time in between to exploit here. For instance, we know that gohan started his mystic power up very shortly after goku went back to the afterlife (as he immediately tracks down gohan and breaks the z sword). Between goku leaving, we have goten and trunks training for a while before super buu arrives. We also have the time he was at the lookout, we have the time that they were in the time chamber, we have their fight INSIDE of the time chamber, they're stuck there for at least THREE full fusion cycles (3 hours at least), we have the rest of that fusion cycle, and then i'm pretty sure they ditch majin buu for some undisclosed time i think.