The Tories have snookered themselves with ridiculous policy at ridiculous timing. But Truss is a stubborn, simple minded narcissist who doesn't want to U turn only one week into her premiership. It has feck all to do with Starmer.
Thats not really true though is it? With a left wing opposition they'd have said corporation tax should be increased to more than Sunak's 25%; leaving Truss a Get out of jail option to fall back on Sunak's 25%; whilst not being seen as anti business (the lesser of two evils)
Likewise a Corbynite would likely have been against the £1m AIA which could well have given wiggle room for Tories to reduce this back down to maybe a couple of hundred thousand to raise some cash.
A more socialist opposition would likely also be against the U-Turn on NI given that this was intended to fund social care which would be a priority for a left wing government who would favour higher taxes to allow higher spend.
Truss would also be able to attack a more left wing labour party who disagreed with Stamp Duty cuts in favour of social programs as an attack on first time buyers.
A socialist government would also have to explain what additional taxes they would levy to fund nationalising rail, energy, mail etc in an environment of expensive debt and wobbly confidence. Starmer has merely made a Johnsonite suggestion of a few billion for an energy quango.
A left wing party would also be vehemently against a points based immigration system (I agree with that by the way) and would have fallen into the trap of being able to be portrayed as a "low wage, high immigration" party.
Starmer has avoided all those traps by merely agreeing with the vast majority of tax decreases and agreeing with the vast majority of spending commitments.
He's basically marketing himself as to the right of Sunak economically (he agrees with £100b to cut corporation tax, income tax, NI and freeze energy prices), but a little to the left off Truss (he disagrees with £2b of higher rate cuts).
Throw in the odd populist Johnsonite focus-group approved policy like an(other) windfall tax and he's sorted.
Starmer is basically who the Tory Parliamentary Party would have voted for if it didn't go to the members.