For example, try saving my home page: https://andrewrondeau.herokuapp.com/
The img tags are converted correctly, but there's still <figure class=image><a href="https://andrewrondeau.herokuapp.com/... in the single HTML file.
Even better, just search for ="http
Edit: I think I now understand the issue. I confirm SingleFile saves only the current page and not linked images, for example.
For example, try saving my home page: https://andrewrondeau.herokuapp.com/
The img tags are converted correctly, but there's still <figure class=image><a href="https://andrewrondeau.herokuapp.com/... in the single HTML file.