So you head out to that link on the Export menu, right-cilck and save the JSON which then you can upload to the Lighthouse diff tool.
But there's more. You don't have to save any files, you can just link to them. Not sure if this is documented anywhere but the diff tool takes
&compareReport= like so:
https://googlechrome.github.io/lighthouse-ci/difftool/ ?baseReport=BEFOREURLGOETHHERE &compareReport=AFTERURLGOETHHERE
You can use the WPT URLs and pass them to LH diff tool, even automatically if you use WPT API.
Just don't forget to escape the URLs, e.g. in the console like:
> encodeURIComponent('https://www.webpagetest.org/lighthouse.php?test=230510_BiDcPD_2R7&f=json') "https%3A%2F%2Fwww.webpagetest.org%2Flighthouse.php%3Ftest%3D230510_BiDcPD_2R7%26f%3Djson"
Otherwise the & in
&f=json in the WPT URL will cause widespread confusion.
Finally, an example URL with everything setup just like it should.