Blockchain doesn't provide irrefutable evidence of anything except that the record wasn't tampered with after the fact (and it's a very expensive and wasteful way to do so). Blockchain does nothing to verify the correctness of the data it holds, only its consistency with the history.
You would need control of 51% of the nodes to submit a block with arbitrary data and override consensus.
You can’t just add any hash to a block and add it to the network. In order to be the miner to add the next block, you have to win a competition to find a correct hash that solves a difficult math problem. Since there is no way to start with a resulting hash and work backwards to figure out what piece of data gave that hash, the Bitcoin protocol uses this feature to create its difficult math problem. The math problem stipulates that the first miner to produce a hash with a certain amount of leading 0s will be the winner of that block and be able to add it to the network.
>You would need control of 51% of the nodes to submit a block with arbitrary data and override consensus
This is only true for corrupted miners. That is not an issue in this case because the company controls 100% of miners (and therefore, you are trusting them not to write bad data).
It is completely irrelevant to the question of corrupted clients, which no blockchain can guarantee against. All you have to do is attach some bypassing wires to the input and output of your "blockchain-enabled smart meter" to write completely fraudulent entries to the history that can never be fixed.
