That's only because they have Luke Faulk, the third stringer, in the game. Both Darnold and Siemien (the backup) went down. Not surprising the stats are that bad. If Darnold was behind center, I assure you the stats wouldn't be that bad.
Also, the reason Le'veon hasn't had 3+ yards (average) a carry is because the 4 teams they've played have had stellar defenses. Patriots and Bills have top 5 D's, Cleveland is more middle of the pack but Myles Garrett is a monster, and Eagles have a terrible secondary, but an okay Run-D which was all they needed to focus on with Faulk at center with no real WR threats. Dallas does not have this privilege. This game will be closer than you think.