The fact is that they are rebranding it to Steam Deck OS.

As far as I know they've said nothing about that. It's reasonable to say that the title is a typo for a couple reasons.

1. They've stated they want SteamOS on third-party devices before
2. There's also the rumored VR headset running SteamOS as well

Two use cases for the OS that have nothing to do with Steam Deck, so "Steam Deck OS" makes no sense as a name. I personally think they are waiting for Plasma 6+HDR support+VR support before they ship a desktop version, it's important to have feature-parity with Windows out the gate for good word-of-mouth.