^^^ To be fair to AB's techies, O_G, there are lots of different browsers, across lots of different devices. So they can't test the website on all possible combinations.
I'd expect them to check the website's functionality using Chrome, Edge and Firefox on a PC. On Android devices it would probably be best for them to check it against Chrome, Opera and Samsung Internet Browser (remembering that there are actually different versions of the site for mobile phones and tablets). With iOS, they ought to check the site with Safari. However there are a vast number of different browser/device combinations that visitors to the site actually use, so they'll never cover all of them.
[If there is a big weakness with their testing, it's probably with iOS. iPad users seem to experience far more problems with the site than anyone else does].