This isn't actually true by the way. Not that you can trigger amanthuls even if there's only amanthuls, but the "every player with an active normal proc" thing.
To demonstrate, take the mythic garothi fastest kill (picked that because I assume that raid has full pantheon trinkets).
Here's a 30 second snip of the NORMAL pantheon trinket uptime, for the agility users from 30 seconds in, to 60 seconds in:
https://www.warcraftlogs.com/reports...4675&end=72727
Here's the same timeframe, but for the pantheon proc:
https://www.warcraftlogs.com/reports...4675&end=72727
You can see the first pantheon proc happens at 40.5 seconds. At that point in time, both the demon hunter, monk, and the rogue Skyze had a normal proc up (the enh lost his buff .4 seconds prior to the pantheon trigger).
If what you said was true, then they should all 3 have triggered an instant pantheon proc at 40.6, but it's only the monk that does - both the DH and rogue that had it up at the correct time never triggered a pantheon proc in that timeframe.
Conclusion: When 5x buffs are up at once, pantheon is triggered. However, it will pick a random pantheon-user to grant that buff, if multiple users are present (EG; 3 Agi, 1 int, 1 healer, 1 tank = random agi of the 3 gets empowered).
Simply put, if you've got 8 agi 4 int 2 str dps in your raid, you're probably getting fuck all out of the pantheon proc as an agility user. Meanwhile, the 2 str dps will be fucking swimming in procs.