Post seems confused. A 51% attack doesn't allow the attacker to sign transactions with someone else's key.
Maybe you misread, the post says this: "With its current dominance, Qubic can rewrite the blockchain, enable double-spending, and censor any transaction."
All of which are possible if someone has that level of control, and none of which involve signing with other people's keys.
(As some people seem confused about the impact of 51% attacks: Of course you can't double-spend in a single blockchain, as that is prevented. But the nature of these attacks is that there's no longer one true blockchain. You can create one fork of the blockchain where you send the money to someone, receive goods in return, and then afterwards switch to a longer fork of the blockchain where the money was never sent.)
This doesn't seem like as much of an actual risk. A better way to make money would be to create a perception that the value of the coin is at risk before buying it cheap.
Actually devaluing it doesn't seem worthwhile financially.
I have an idea for a much cheaper way to store and transfer money that also relies on the existence of a police.
Why would you do this double spend attack with goods (and I assume you mean physical goods) and not for example a swap to ETH?