I'm going to answer my own question because I've received some feedback.
The reading coach feature is not supported in the Lens app, nor will it be supported in the near future.
However, I received a link where I could post it as a suggestion, which I did of course.
You can find it here: https://feedbackportal.microsoft.com/feedback/idea/4f71c19d-b75a-ee11-a81c-000d3ae46fcb