How does snapchat know exactly who I know?

I went on a cruise ship a while ago and I met someone in the last few days on it. I never had any service (since we were out at sea.) When i was able to connect to the internet, i friended them on Discord. we never shared any phone number, no airdrop etc.

I live in Australia, and the other person lives in Texas. Again, we didn't ever message each other, let alone add our phone numbers. Just discord.

And about three months later, i get a Snapchat notification, recommending to add the same guy i met on the cruise ship, without having any mutual friends or phone number details.