It's possible that their accounts were grandfathered in and already marked as "different people". During the next little while (the exact period is unknown, but let's say 6 months), any new accounts from that IP may get banned, but especially those that are deemed "suspicious".

We don't know the exact details because Reddit doesn't release the details. You might find someone here who can tell you about workarounds, but most likely you'll get the advice to move on and find something new. Most people are here because they no longer want to use Reddit, and I'd say the vast majority is here because they chose to leave and not because of bans

they often use browser fingerprinting and getting various device settings. it turns out that just by looking at various configurations and such, you can actually track individuals