I’m extremely skeptical of your claims that you learned Chinese grammar in a week as a Chinese learner myself and I’m willing to bet you don’t realize how much you don’t know. The Chinese grammar wiki has 505 articles on grammar split across A1-C1 levels of the European Common Framework for language proficiency. This wiki is also non-exhaustive. This isn’t even including the fact that Classical Chinese, which is a basis for many 成语 used today, has a completely different grammar than modern Chinese.
It might not be technically correct that they learned the full set of Chinese grammar in a week, but I can imagine one week is sufficient for a beginner to get the basics and start reading/speaking/writing the language and absorb the other nuances or nitty-gritty details through everyday usage.
I mean, I'm pretty much a Chinese/Cantonese native user, so I can't be sure about that since I never "learned" the language in a classroom setting, but my impression is that this short period of learning grammar for Chinese language leaners is quite typical.
