The source is reportedly a Ukrainian black market dealer, who somehow got a hold of prerelease stock of the Pixel 3 XL, according to 9to5Google. The publication traced the leaked Pixels back to a Telegram account, where an anonymous person is allegedly selling Pixel 3 devices for $2,000 a pop.
It's unknown how this seller obtained the unreleased phones, how many they sold and how many are left. Due to the sketchiness of it, 9to5Google originally wondered whether the seller may have simply been scamming potential buyers.
The report also links the seller to a Wylsacom, who could be the same Wylsacom responsible for some of the leaked Pixel 3 XL pictures. Perhaps that's where they got the phone in the pictures (not left behind in a taxi like the tweet claims).
After purchasing a phone, the seller reportedly allowed buyers to pickup the device in London, which is the same city Instagram user Khoroshev posted pictures allegedly taken by a Pixel 3 XL. Coincidence? Maybe, maybe not.