As far as I'm reading this, it says'A creature can’t gain temporary hit points from this feat again until it has finished a short or long rest'.
It doesn't say you can't use it on anyone else per short rest.
What am I missing?
Nothing. You can certainly be like the medieval king going round the campfires of his troops before the battle, talking to and inspiring them in groups of six... until they finish a Long Rest. Not exactly overpowered IMO; you can only inspire 36 men per hour, 216 in 6 hours. Could be very useful for a Magnificent Seven scenario where you want to inspire the peasant village to fight off the brigands, but it takes too long and affects too few targets to do an 'Aragorn at the Black Gate' thing.
Edit: Personally I'd be inclined to house rule it to include some lesser battlefield-wide effect, eg
"at 11th level with one use of this ability you can grant temp hp equal to (1+your charisma bonus) for the next hour to every ally who can see and hear you" - plus a bonus to actual morale, ie they won't run
away.
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