They used a small team to build an app in 5 months that handled going from 0 to 100 mil users in 5 days.
Doesn’t sound neither lots of money and engineering, or false idea if it had 100 mil signups in 5 days.
At 1.2 billion monthly users that's a pretty average CTR for that kind of thing. A little extra because threads still isn't available worldwide so some traffic was just lost.
They didn't have to, though - the news outlets were running stories on Threads and its impending arrival nearly every day. To claim, as GP does, it was entirely "organic" is nonsense.
Threads reached 100M users in July [2].
[1] https://www.insiderintelligence.com/content/threads-banks-on...
[2] https://www.forbes.com/sites/siladityaray/2023/07/10/with-10...