Do I need an argument? The guy has no credibility.
Yes, you need an argument. Regardless of your opinion on CSW, his points stand on their own.
And it's not like this is some new oddball claim he is discussing. It is an aspect of SegWit that we have known since inception, bot for some reason never gets discussed.
Without SegWit, the most devastating thing a miner 51% attack could accomplish is to roll back some transactions. In all their expenditure, they are unable to claim others' coins for themselves.
However, once SegWit activates, the attack surface of a 51% attack takes on a new aspect. If a miner (or a cartel thereof) accumulates 51% of the hashrate, they are able to steal the funds of others. All it requires is mining new non-Segwit blocks that contain transactions which spend the 'anyone can spend' transactions in previous SegWit blocks.
Further, every additional block for all eternity contains additional incentive for the chain to be so attacked, as it will contain monotonically-increasing value locked in transactions that are SegWit under the new rules, but subject to theft via anyone-can-spend chain rollbacks.
So CSW aside, how come nobody wants to talk about this aspect of SegWit?