Author here. It's not exactly that; the <img> approach is commonly suggested, but it has some serious limitations (explained in the source code).

Instead, I time <iframe>s, which allows SOP violations to be trapped the moment the browser barely starts thinking about rendering the target page. The other benefit is that <iframe> requests can be aborted quite easily when they are taking long enough for us to suspect cache miss - before the request is completed and cached.

The results should not be fuzzy, although the PoC uses hardcoded timings instead of doing calibration, which makes it a bit tricky with "outlier" clients (very fast or very slow).

I made some minor tweaks today, and the success rate should be greatly improved; there's now a mini-survey on the page, looks like ~90% of the people who bother to complete it are getting accurate results.