If I would be a stranger who would like to learn Czech, I would never use duolingo. Its not posible to learn anything because its not grammatically right. They dont describe nor teach the difference between díky and děkuji and děkuju so finally you think its normal to say Ahoj when you enter somewhere and say díky when they bring you a tea. Its ridiculous.
I can assure you that it IS often normal to say "díky" when they bring you a tea. Maybe not when you want to be super formal, but it is a normal thing to say.
And here it is a translation of "Thanks!", that is not super-formal either.
And last, this has nothing to do with grammar, this is a style and usage problem.