http://www.penumbragame.com/buy.php says this:
Linux Minimum Requirements:
Same hardware as Windows
Kernel 2.6 or higher(2.4 untested)
glibc 2.3
X11R6 with 3D acceleration
For x86_64 CPU, 32-bit environment
must be installed
As for the little note at the bottom of the purchase page, do you seriously expect that such is enough, given that the game will not work whatsoever if you do not meet a very narrow set of requirements, which aren't made clear?
Perhaps you should update the Linux system requirements stating exactly what graphics system you need, or which ones do not work.