How? "Screenshot" is used correctly by the OP.

For reference:

  screenshot
    (verb) an image of the data displayed on the screen of a computer or mobile device.

Their usage does not match that definition. When recognising writing, they are recognising an image of the page, not an image of what was on the screen at some other point.

Also, your definition describes a noun but claims it's a verb.

The promotional material shows the application interpreting a screenshot of an equation, not handwriting.

I also meant noun, not verb. Thanks.

I see absolutely nothing wrong with the usage here.

Screen grab?

Photograph or picture, surely?

You should reread OPs paragraph. Also if you go straight to the website it shows you the desktop app first which is using screenshots from whatever to turn it into Latex.