Can Icefangs not run? or Is the Icefang we see in Moon 6 injured?
They can run, but not far!
Smilodon is an extremely bulky animal, especially compared to weirdly gracile Homotherium. I like to imagine the Ice Fangs as much more ambush-dependant, lunging out and either getting a good grapple or having to hide again. Compared to the Fleet Fangs, which can trott along nearly effortlessly, and have a very efficient loping stride; Burnet isn't breaking a sweat where the Ice Fang is spent!
Smilodon running by Mauricio Antón, note the lack of suspended gait!
This is part of the reason I'm not a fan of Moon 6.2; the distance seems way too short, but I had intended the Ice Fang to have chased Burnet across a large section of Steppe before losing her.
This wasn't clear from the art though, so that's my bad >:/