This is what the ingame notification message reads:

"We have confirmed with Twitter regarding their recent announcement on API access that the policy change in API will not affect players who have bound their Twitter with the game account. Nonetheless, we strongly recommend binding your account with the Yostar account or any other third-party account in case of contingency."

It's a lot clearer and shows you can bind with the other methods. (Googleplay store, Facebook)

Also, this tells me they will be paying for the API but are also preparing to pull out (which makes sense, twitter is very volatile atm).

I've made a comment earlier but accidentally deleted (thanks reddit mobile):

Basically I said the more successful gacha games and in-app purchasing titles will likely keep the binding. No one wants to lose a not-insignificant segment of their current audience and reach. The more successful the game, the more it would actually cost to not pay the API fees.

What will cause companies to pull out is how unpredictable twitter is atm. If they implement less favorable policies or make it more difficult to work with, it may very be worth moving away, even with the audience loss.