Codenamed Piston, Xi3 began taking pre-orders for the 'Steam Box' at the South by Southwest festival over the weekend.
However, despite previously backing the machine, Valve is now distancing itself from the device.
"Valve began some exploratory work with Xi3 last year, but currently has no involvement in any product of theirs, Lombardi told Eurogamer.
The statement comes after Xi3 released a trailer for the console/PC hybrid, showcasing some of its features (above).
Unveiled at this year's CES event in Las Vegas, the pint-sized system started life as an unsuccessful Kickstarter project, before gaining the backing of Valve.
Designed specifically to support Steam and its Big Picture mode, the Xi3 Piston will cost upwards of $999 (£670) and ship in time for the 2013 holiday season.
It is said to contain 1TB of internal storage and offer modular component updates, including the option to upgrade the CPU and RAM.
Valve co-founder Gabe Newell previously revealed that the company is working on living room-based hardware, and has since confirmed that it will release its own Linux-based 'Steam Box'.
Valve took its first step towards optimising Steam for the living room with the release of the Big Picture mode update, optimising the service for TV screens.
Watch a video introducing Steam's Big Picture mode below: