a valid block is simply by definition, that block on which miners decide to build the rest of the chain.
This directly contradicts the text of the whitepaper-- which specifically talks about an attacker overpowering with invalid blocks:
As such, the (simplified) verification is reliable as long as honest nodes control the network, but is more vulnerable if the network is overpowered by an attacker. While network nodes can verify transactions for themselves, the simplified method can be fooled by an attacker's fabricated transactions transactions for as long as the attacker can continue to overpower the network. One strategy to protect against this would be to accept alerts from network nodes when they detect an invalid block,
But more importantly, it contradicts the behavior of every version of Bitcoin ever released, including all those released by Satoshi. It also contradicts the behavior of every blockchain like altcoin that I'm aware of.