Cool, but at the moment its not that Rex gets TM and the others dont. It appears they all do but Rex gets the most.
Lets focus on this sentence in his unique. "whenever a Clone Trooper or Phoenix ally is inflicted with a debuff, Rex gains 5% Turn Meter per debuff".
We assume that since this is part of his unique, it should apply to other phoenix members aswell and we assume then that for Ezra for example it would be:
"whenever a Clone Trooper or Phoenix ally is inflicted with a debuff, Ezra gains 5% Turn Meter per debuff"
So if Chopper receives 2 debuffs, and hera receives 2 debuffs, we could assume that both Rex and Ezra would receive the full +20% TM from 4 debuffs, since Rex unique is now Ezras unique aswell.
But somehow, Rex gains double.
But if Rex is the only one that can gain bonus TM it isn't clear at all in the description since Heras lead states that all their uniques get shared amongst them.