I think it’s a combination of IP address as well as app fingerprint when you login to multiple accounts within their app - they can see each account coming from the exact same app/device. I personally believe this was a HUGE part of killing off third party apps so they could collect a lot more data directly about you in their own app.

And IP factors in too. I was in hospital WiFi for a few days when my kid was born and got a random message on all of my accounts that I was banned for 7 days for “ban evasion”. Never been banned before and had only posted on some parent subs during that time.

I think the IP address probably had bans associated with it given it was such a huge public network and was next door to a high school that used the hospital food court a lot too, and thus the WiFi also.