Technically yes a console is a PC but from an end-user POV the difference between a console and a "PC" is one is closed and one is open (open as in upgrade hardware, install whatever software you want, even a different OS).It sounds like SteamBox will fall into the "open" category so it is closer to a PC than a console.
Which is great I believe but now they are competing with a slew of existing "gaming PCs" with no clear advantage that I can see and none of the true console advantages like being a fixed, stable platform for 5-10 years. I mean, people will be upgrading/modding/adding different controllers (requiring different drivers) all the time to this SteamBox so it won't be stable like a true console. Meaning even if a game is designed "for SteamBox", it still will need to be tested on a range of variable hardware/software. At that point game developers might as well just target Windows/Linux in general.