I don't like Starmer, his party, his politics, or his policies.
But. BUT.
If he DIDN'T loudly shout "no open cheque book" at this point, but promised to fix funding shortfalls, it'd give the Tories carte blanche to run up huge deficits via crony outsourcing contracts and then dump the resulting mess in Starmer's lap—and him having just promised to fix the spending shortfalls. Which would kneecap his first term spending plans.
Similar with "we will persist with Brexit". It's nonsense, but ...