An important point in this is that at 15% the MM might give codes to suggest a number of tasks in addition to changing the oil. However, there is only one reset procedure which resets everything as being done. Thus whether you are at 20% with no codes showing or 10% with transmission fluid change, replace rear differential fluid, change filters, etc. showing, if you change the oil (or even if you don't) and reset the MM, the computer 'thinks' ALL of its recommendations have been followed and does not remind you at the next 15% point. So at 60,000 miles the MM might recommend changing the oil and the transmission fluid, but if you only do the oil and reset the MM, you probably will not see a transmission fluid code again for another 60,000 miles.
If you ALWAYS change the oil at 20% and reset the MM, you will NEVER see a code for anything else. IMHO that is the significant point of what you were told - and a weakness of the MM design.
I think each recommendation should have to be individually reset - you could still fool it, but not be unintentionally mislead.