"Si oui, tant mieux !"
I guess we have phrases in English as well... "and so much the better" is more of a phrase. ..."so much better" doesn't really sound write in this sentence, even though it means the same thing
i guess there are still things being improved with this program." If yes, so much the better" should be accepted. It's a direct translation. There's only so much one can expect from learning this way. I still think it's fantastic.
"Si oui" appears to be a colloquialism used in certain contexts. I propose Duolingo update the definition when one mouse-hovers over the pair.
Actually, it's perfectly legal English. Being lazy, I'll just point you to the Web:
"if so, so much ¿THE? better" It'd be perfect English but sure sounds like hell and I've never heard it like that before in my life...