It was more the letter number combination that Intel had been using consistently for consumer motherboards for over a decade. Then with Zen, AMD co-opted all the Intel consumer platform naming schemes. Not the first time either, they did similar with CPU naming conventions before Zen.
Intel Z370, AMD X370, X470, X570. Not to mention Intel's X platform which Threadripper also co-opted. Though Intel didn't give in as much there. Intel X99, X299, woops AMD X399...
So Intel moved to using Zx90, There hasn't been a follow on HEDT chipset since, but Intel looks to be doing X699 and AMD seems to be done with consumer Threadripper and is switching their WRX80 chipset.
Adopting Intel's 3/5/7/9 convention was a good move. Let's people have a consistent way to tell what level of CPU they are getting. All AMD had to do was come up their own distinct chipset names. They probably could have stuck the A convention they had going, T for Threadripper. They had options that wouldn't have involved fighting for a trademark name.