GraphQL doesn't solve those problems, it just makes it less work to implement the transport of data

seems like the graphql schema can be a good enough proxy

My experience is that people assume they don't need contract tests because of graphql tooling, and then they have issues in prod because the tooling couldn't detect the issues.

Of course, you can use what you like. You don't even need to test at all, as tests are only risk reduction. However, doing contract testing (whether or not you use pact) is more than just "is this change a breaking change to the schema". For example, just because an update is backwards compatible it doesn't mean that a non-updated provider is compatible with an updated consumer.

Can you tell me more about your setup? If there is tooling that provides proper deployment confidence for GraphQL, I'm very keen to hear about it.