Yes, Sony are a 3P to Valve and I merely listed the reasons. One of those is a....pretty BIG reason, i.e SIE bringing virtually all of their games from this decade to Valve's Steam storefront.
The reality of the market is one where SIE simply can't say the same of Valve being a 3P to PlayStation, because they aren't. None of Valve's peripherals or systems are compatible with PlayStation. None of Valve's modern releases from this decade or really anything beyond the Orange Box for PS3 have been ported to SIE consoles.
But from Valve's POV, SIE are a 3P, and they'd be correct. They're a 3P who just happen to have a platform of their own (PlayStation).
I'm only bringing this up to say, the same can't be reciprocated from SIE WRT Valve. They can't claim Valve are a 3P who just happen to have their own platform (Steam). Especially considering, SIE are much more dependent on Valve's platform for future growth than the other way around :/