Controlling 51% of XMR costs ~$30M per day, you'd have to short a huge amount of XMR to make that worthwhile. Who would be the counter party and how would you do that anonymously?

The attack itself is unprofitable, the "profit" for Qubic is the publicity they get. (or at least that's what they're betting on)

Monero has a theoretical market cap of $4.7B USD and daily volumes >$100M USD. I wouldn't recommend taking that short position in one go but over a few days and a few exchanges I wouldn't see a problem acquiring a very large short of the token.