Shorting Bitcoin has been possible for a decade now. The capex to pull off the would be in the billions though, and the value of that investment is tied directly to the price of Bitcoin.
That said, the attack you describe has happened for much smaller cryptos. I'm not saying it can't happen, I'm saying there's no reason to assume it would be a huge threat to Bitcoin, because the actual risk for a user is vanishingly small. There are much bigger threats to Bitcoin's valuation that are far more plausible, such as government crackdowns.
The question, and this thread, was entirely about if it this is enough compute to do it. Not if it was, in your opinion, a threat worth worrying about.
