Even to the extent that it might be a mistake on Gromit's part to call it a "lie", it would hardly count as an example of illiteracy.
But the real problem is that it's switching the blame to the wrong people. There is every sign that the pledge will fall well short of that intended goal. See eg
https://www.theguardian.com/society/2023/feb/04/only-10-of-boris-johnson-promised-40-new-hospitals-have-full-planning-permission
Even if it's not a lie, it's an empty promise that is bound to fail. I don't mind politicians setting ambitious targets, but I *do* mind it when they find cunning and deceitful ways to argue that they didn't fail.
It's the same with the bus. The £350 million figure was plain wrong anyway (see eg the multiple ONS statements debunking it), so that's already misleading on its own. Saying "Let's spend it on the NHS instead" may not be a literal promise/contract, but it deliberately misses the force behind a "Let's" statement, which is a clear and unambiguous statement of intent. If that statement is then not followed-up, it's reasonable to feel aggrieved -- both those who agreed on the understanding that this would happen, and those who were frustrated that this sort of empty promise ended up working.