What is so difficult to understand here? Invalid blocks are not in the blockchain. The only way to add a block to the blockchain, is to mine a valid block. Those who produce invalid blocks never actually have any say as to what makes it into the blockchain.
Who can decide whether or not an invalid block makes it into the blockchain? Who determines whether a block is valid or not? Isn't it the miner who mines the next block? They decided to mine on the last block so they say the last block was valid, no one else's say matters. Then the only thing left is do is either the miners mine on that last persons block, or say it is invalid and remine the previous block. No where in this process are non-mining nodes. They are not part of the block confirmation process.
The problem with segwit is that the block will appear valid and can be validated in the blockchain without the signature being present. This is the whole point of segwit, so that the signature is not needed in the blockchain. In order to see if the signature is valid the miner would have to wait for the witness block before they start mining on the last block which takes extra time.