You ought to do some basic research before making a post like this. Honestly might be the most baffling comment I've ever seen on HN---what kind of mindset does it take to have this kind of overriding confidence in one's own lay speculation?

The engine bells are made of different material than what's behind them (a material that has to directly withstand hot exhaust) and the aerodynamics of the ass end of the rocket are complex; some combination of these factors means this heating isn't an issue for the bells but is a concern for what's behind them. The booster does not re-enter from orbital velocity, but does come down engines first at supersonic speed. Stability in this orientation throughout the descent is a problem SpaceX solved with Falcon 9, and SH works the same way.

See the other reply to your comment for timestamped video of aerodynamic heating behind the engine bells.

Is it possible you're confusing the booster with the second stage (Starship)? They are not the same thing.