I was not aware that the error bars between kashrut and marit ayin are that wide. I figured that it was because of the intersection of Jewish middle class culture and Chinese culture starting in the 1930s, and because Chinese restaurants generally do not serve dairy products.

I think those are factors as well!

This is all apocrypha, so take it with a grain of salt. But my understanding is that this would be a case where marit ayin would not be a significant concern, since the Chinese dishes in question were not visually identifiable as e.g. pork.

Or another framing: if you were a semi-secular family (like mine) that tried to keep a semi-kosher home, it would be easier to eat a dish that contained finely minced pork or shellfish or similar. American Chinese food fits those parameters while also being available on Christmas, etc.

I was under the impression that Chinese food is all about pork. Something like that Modi's rant: https://m.facebook.com/reel/1021462526286349

There's also a lot of vegetarian/non-pork-but-fleishig Chinese food. But the point was more that Chinese food that isn't kosher isn't obviously so, especially 60-70 years ago when it was less commonplace.