> Reverse engineering for the purpose of interoperability is explicitly allowed.

In Apple's iMessage TOS? I don't find that likely but open to being wrong.

> Also, breaking the TOS is usually not illegal

In general, contracts are legally binding, therefore breaking them is illegal. Sometimes contracts include clauses that can't be legally binding, but I don't think a TOS forbidding this type of behavior would be questionable in the slightest. Apple obviously has no obligation to allow anyone to use its platform as a backend for their own (previously commercialized) product.