It's on the high side, but...honestly not absurd? "Party" implies one night rager, but the source says "in-person company event." That seems more like a multi-day company onsite to me, and the total bill per person there probably includes travel, accommodations, food on top of any overall event costs.
Bringing a remote employee to SF just to work out of an office for a few days can easily cost a few grand.
airfare: $500, 5 nights (sun -> thur) in a hotel, fully costed: $250, per-diem of $100 6 days (fri is a travel day) is already $2350. If you rent a place that thousands can show up you'll be in for at least $5k.
I've been to all hands where it probably cost that much just in travel: business class LHR to SFO, hotel for a few nights, dinners, drinks, entertainment, venue, guest speakers, and on and on.
It doesn't seem excessive, the networking in these things is often really worth it
Not everyone, but often there are travel policies where any travel more than X hours long you can book business class without manager authorisation via concur and the like.
It's really worth it when you get off a 12 hour flight and have to go straight into hours of meetings without being a zombie
Bringing a remote employee to SF just to work out of an office for a few days can easily cost a few grand.