This can be explained as whales/institutions telling BFX "I want $100M of BTC" and a human OTC trader at BFX executing it optimally.
Remember this is newly printed tether which means net deposits of new actual usd, if you believe them - and of course I do not.
In the time since it was published, when Tether had issued about $4.5 billion total, there have been over $14 billion additional Tether, a 4X expansion of the total supply, or 7.5X what was described as "large swaths of Tether" in this article.