That's an extraordinary assumption, and the kind of thing which is amusingly being peddled by the losing Tories as a desperate attempt to scare folk away from him.
What's fascinating is how little the modern Labour party is understood. For Keir to become a "puppet", there would have to be strong and scary puppet masters in the background.
The actual reality is that if Keir becomes PM, the same setup we see at the NEC and conference, will continue. Many frustrated Left wing activists will bite their tongue and fall into line for the promise of power or patronage. Others will mouth off on TV, and find their influence wane etc.
There isn't a shadow government or a Left-wing cabal waiting to pull Keir into line. Please can people stop peddling this stuff... it's just weird.