I have no LinkedIn account. I cannot view the article (nor anything else) on macOS in Safari and Firefox, neither normal nor private mode, and it has been so for months, possibly years. I can view the article in both normal and incognito modes on Chrome.
Coworker 1 is on Arch, has a LinkedIn account but has never logged in on his machine on any browser. With Firefox he can only browse a couple times before the wall kicks in, until he throws cookies away (manually or with private mode). About 15 min later, he is locked out and cannot see the page, even in private mode. Clearing every single bit of data from his Firefox profile does nothing at making the page accessible again. Still, Chrome displays the wepbage fine in both modes.
Coworker 2 is on macOS. Using Chrome he can see the page in normal mode (he had previously logged in). He cannot view the page at all in Incognito, ever.
We all share the same IPv4 but have IPv6 enabled.
It has to be some wickedly twisted heuristic possibly involving some evercookie.