They couldn't become an Instagram because they knew their growth wasn't sustainable or real in any way. They had to sell or they'd fade into obscurity.
New distribution channels don't appear that often and when they appear you should rush to be early and aggressive, as they decline in effectiveness over time, which limits competitors' ability to match you.
Of course a strong product and good retention are essential to keeping those users, but you are a video app and this opportunity were to present itself you would be crazy (stupid) not to jump on as quickly as humanly possible.