I agree - the link isn't useful. I've found the same issue this morning on my (mid-2011) 21.5" iMac with High Sierra. It's just the purchases tab that gives the message, even after signing into the store again. I can access my account and, for example, see subscriptions but trying to manage hidden apps takes me back to the tab and "Cannot connect..." message.
This seems like Apple trying to get users of still good hardware to junk it and upgrade to a new M1 (something I thought about but discounted as my iMac still works great - (I'll admit is was maxed out when I got it - 256SSD, 2TB HDD, i7 CPU, 16GB RAM). Being stuck on High Sierra isn't a problem in itself as it's a good, solid OS; I prefer it to Big Sur on my (mid-2014) MacBookPro, and it runs a lot faster than my wife's late-2013 iMac on Catalina. My only issue is that Apple has dropped all support, and even Microsoft has dropped it - Office365 no longer updates (and, I'm told, won't install if I needed to reinstall it), and OneDrive support stops next month, too. If I have to ditch my iMac because even basic services are withdrawn, I'll hook my MBP into my desk system (hub, monitors, external drives) and use that... It should keep me going for a few more years.