There isn't enough computing power available to 51% attack a chain that comes close to justify the $1 billion dollar "bounty" on this transaction.
The miners who control the bitcoin network are already centralized on a handful of organizations in China. If they colluded on this attack they would take over >51% of the network because, hey, they already own and operate most of that computing power you're eluding to.
Edit: There is also no evidence that the price would plummet from this, and given the history of theft, scams, forks, etc. involving bitcoin that do not drastically affect the price, I don't think it would either.
When I saw the initial math, they said 40 days. But that doesn't include the fact that as you buy hash power, it's price would go up. But to secretly mine for a week before reorganizing the chain would be very possible with this amount of money.
I wonder if you could put in a checkpoint system. And if that could even be decentralized.
When I did it the network hashrate was 44,000,000 TH/s and if you could buy enough S9i's to the same low price, it would cost you around $650 million. This is excluding any other infrastructure you'd need and of course the power you'd use to perform the attack. And since I did the calculations the hashrate has -doubled-, making it cost quite a bit more than a billion.
Now maybe you can get off cheaper if you try to rent the hashrate. Around $18 million of BTC is mined per day, say you can get 50% for $10 million per day. So yes maybe you can do it for 40 days, if you want to risk a large part of that billion. And face the quite massive risk of tanking Bitcoin's value or facing devs who might launch a "protective hard fork".
(This is assuming you -can- buy or rent enough hashrate, which is very uncertain).
There are checkpoints in BTC, although they haven't been added since 2014.
[0]: https://whycryptocurrencies.com/how_do_cryptocurrencies_work...