It's like when someone says that Romo has only thrown 7 interceptions this year, which ranks him 3rd in the league. But when you look at each one, 5 (or whatever) of them weren't Romo's fault for one reason or another, so he really only had 2, and that would rank him first in the league.
Well you can't say that about Romo without also going back and seeing how many interceptions were not the fault of all the other quarterbacks out there and then go back and rerank all of them.
By the same logic, you can't say that Emmitt ran behind Williams and Allen without also saying Sanders ran behind Lomas Brown, and Alexander ran behind Hutchinson and (the other guys name is escaping me at the moment), and Dickerson ran behind Slater, and Riggins ran behind Jacoby and Grimm, etc.
So the question is, was Emmitt's line during his career SO much better that his accomplishments should be held out as a product of line play more so than any other great RB?
I say no, because (a) his line wasn't all time great for more than 5 or 6 years of his career, which is a minority of Emmitt's total seasons, and (b) even when his line was elite Emmitt wasn't going off for 2000 or 2100 yards like you might expect when a great runner runs behind an all time line.
Which means that Emmitt didn't rack up all those yards in just a few seasons while he was behind an elite line; instead he got the rushing record because he was still putting up 1200 and 1300 yards in seasons in his 30s when almost every other RB ever is basically done, and because he almost never got hurt, so he never had that one or two years in his prime when he might've had under a 1000 yard season due to injury. And since he did not play behind an elite line in the last 8 years of his career, the line wasn't what made him.