You actually do not need to know the consensus rules to determine the PoW chain with the most work.
Simply by understanding the hash function and difficulty, you can calculate the total work of any PoW blockchain. This provides some amount of trust that is very difficult to subvert. You only need to know the consensus rules if you want to be a miner on the chain.
If you have multiple implementations that disagree how do you decide which one is the right one if there are also 2 competing blockchains available?
You said "You only need to know consensus rules. You can use any software, you can build it yourself, you can order an audit, you can write software yourself." and that's plain wrong.
You need to ensure you got the right consensus rules. This is not as easy as you put it.