So you don't know anything about gambling lol.

Yes, generally both sides of the book balance (minus a cut for the bookie). But there's not automatically another exactly opposite bet of the same size lol.

I don't think knallfrosch's post was implying it was a singular someone.

Like if you say "someone is at the door", you don't literally mean that exactly one person is at the door.

Saying "someone lost $500k" definitely implies one person. 10,000 people losing $50 isn't noteworthy.

And someone at the door? If you see 3 people, you don't say someone. You'd only say someone if you're expecting it to be one person (which it usually is knocking at a door).