AITA for basically snapping at a stranger who spoke to me in Mandarin because of my appearance?

I'm a 25M Chinese American - I have familial roots in HK (thus I look Asian) but grew up in the US. Also of note is that I recently finished a masters degree in Germany, speak adequate German, and am back in my US hometown for two weeks.

So, I'm in line at Starbucks when a white guy gets in line behind me, gets my attention and asks me something in Mandarin with a smile. Which I don't understand. When I stare at him blankly he repeats himself, then asks me in English if I spoke Mandarin. It felt like a bad pick-up attempt.

Besides being morning-grumpy, I've heard stories from female friends of mine of getting approached with Ni Hao or Konichiwa just because they were Asian, the majority of which were crude pick-up lines. And personally I take poorly to the assumption of someone's ethnicity and culture when in the US; English should be the only one assumed IMO. So in that moment I not-so-politely responded in German "Ich spreche kein Chinesisch. Sprichst du Deutsch, da du weiß aussiehst?" (I don't speak Chinese. Do you speak German, because you look white?) I'm assuming he didn't understand any of that, but he muttered "sorry" and I turned back around and ordered my drink - in English.

I didn't think much of it until now, and while all I did was mirror his own actions (assume his mother tongue) I'm wondering if this was crossing the line. Thoughts?