People read the notes wrong and thought it meant you couldn't have more than one T6 materia. You can stuff as many as you want into open slots on a piece of equipment, it's just saying that once you start doing forbidden melds the success rate for gluing on a T6 drops to 0% almost immediately. It's a bit of a no-brainer if you look at how success chance goes down based on number of forbidden melds + higher materia tier.