In-App Purchases: small pack, medium pack, big pack, mega pack...
Oh. It's that kind of game.
So click Get (but don't bother to download) so you can leave the author a 1-star review due to IAP in the app store. That's what I just did.
I'm disappointed, I'd have expected a bit better from you rain, although shame on me for not spotting the IAPs in the first place - I've only taken a cursory glance around the Android version as I've been mainly focused on playing matches and figuring out how the game ticks.
I'll confirm, but It's probably cosmetic related. There's this RNG machine where you spend candies to get cosmetic items, like a unique chess board, or a different chess master avatar thing etc... you earn candies by playing and there's also a 'donut' currency but at the moment there's nothing I've seen that has a game-play effect. It's all just eye-candy.
Considering the original mod would have been free of monetisation, I doubt its design could handle a true 'pay-to-win' system without breaking so going down the window-dressing route seems logical.
The closest they've come with regards to paying for in-game effects is this.
The custom pass will let you vote on how to balance new and existing pieces. Considering the effect applies to everyone though, it's hardly pay-to-win either.