Two-phase commit deals with network failure, not change semantics. The message itself (whether "BALANCE=40" or "SUBTRACT 40") isn't something that matters to 2pc.
You're absolutely correct, but change semantics changes how you deal with network failure, there not entirely independent - and I'm aware my example is likely too simple, but typing a better one on a phone is hard :)
You're actually both wrong. 2 phase commit is used for all kinds of things. It doesn't always require a network to be involved.
And you were wrong because your example is not a good example of something not needing 2 phase commit, it was in fact an example of a 2 phase commit. That over-drafting you mentioned, that is deferred reconciliation.
That is not the 2PC I know. From Wikipedia: "It is a distributed algorithm that coordinates all the processes that participate in a distributed atomic transaction on whether to commit or abort (roll back) the transaction". ATMs don't do 2PC. They can't about a money transaction which already happened.
Also wiki: "two-phase commit protocol (2PC) is a type of atomic commitment protocol". That's different from deferred conflict resolution from event log.