How is a monatomic gas going to absorb in the IR anyway? /facepalm)
My guess is that Jack Hydrazine's mention of Argon's use (as a protective - i.e., read: inert - atmosphere) in welding may have thrown him off.
But you're right, of course; even if we didn't already know Argon's absorption spectrum, one could still deduce that it is indifferent to infrared radiation, based on its physical make-up.
Regards,