Side note : it's also why saves are not compatible, just saying.
We use every freaking feature the engine has to offer, and we're pushing its boundaries, going as far as writing a lot of custom code for the inventory system, the interactions between characters, lots of screens, locations, achievements etc etc.
There's a lot going on than just renpy on summertime saga, and if you unpack the game's file, you'll see how much of it is actually just custom engine code.