In Switzerland, it’s illegal to own just one guinea pig. This is because guinea pigs are social animals and they are considered victims of abuse if they are alone

This is true, I checked the second to last time I saw this factoid posted, on reddit back then.

Here are my screenshots from the ordinance from that time: https://imgur.com/a/ZkTyEYq

The rule is defined in the Animal Protection Ordinance (AniPO) link at the bottom of this page: https://www.blv.admin.ch/blv/en/home/tiere/tierschutz.html