The leading example of the article was about Zooom videos, not single frames, and in a school setting you presumably know what culture the students are from, so I don't think you need it to work cross-culturally. And I don't doubt that people can deliberately hide their emotions, but many settings are not adversarial, so there would be no reason to do so. E.g., if you imagine that a teacher is teaching a lecture to a large audience over zoom and is using a "puzzlement meter" to see if they seem to be getting it, then any audience member who does feel puzzled will only gain from frowning and making the lecturer slow down.
In general, I think from my experience talking to people over video chat, some emotional information does get communicated over the video, so in principle computer programs should be able to pick up on it too.