[gpvos]

That site should have some text on it like "If you did not get any errors or warnings while opening this web page, your computer or device knows about the ISRG root certificate and Let's Encrypt will continue to work for you." Currently, this isn't immediately clear to (relative) laypeople like me.

[tialaramex]

That test site, though they do link it in their own announcement was actually created as part of their compliance with root trust programme conditions. Specifically Mozilla's conditions require them to prove their setup actually works (modulo the trust they're requesting) by setting up a web site with certificates that would work once that trust is granted. They were also required to provide example sites with e.g. expired certs so that a Firefox developer could check that does what you'd expect.

The good news is that unlike some of the required test sites, which took a bunch of advanced planning (you can't ask Let's Encrypt's service to mint you an expired cert so the expired cert was produced by requesting a valid cert and then waiting for it to expire and making sure not to lose it...) this test is easy for anybody knowledgeable to manually reproduce in a few minutes, as it's just the "wrong" certificate chain with the good trusted leaf certificate you got from Let's Encrypt. So even if they don't heed your call I'm sure someone else can.