They complement each other. ERC-7802 defines the minimal interface for an ERC-20 token to communicate cross-chain, but it doesn’t specify how a protocol should send a cross-chain message to communicate with such tokens. Similarly, it doesn’t specify how a user will transfer tokens on-chain.
I agree this standard should remain minimal, but I would agree it may benefit if they specify an optional interface for the “token bridge” that depends on ERC-7786. It also makes sense to put it in another ERC.