From my understanding, the first and second parties are the seller (Valve) and the buyer (you). They are the two who have a contract between them. A third party, then, is any party which isn’t signed on the contract. That is, an outsider to the contract.

Any other meaning of second party is probably a back-formation from a misunderstanding of the terms 1st and 3rd party.

Disclaimer: I literally have no idea what I’m talking about