I guess you could estimate the distance by sight. If you really cared, I supposed you could screen shot the play and use photoshop to measure distances between hashmarks and base the player distances on that sort of standard, but I have no idea who would do that.
It's literally a garbage stat. PFF made a name for itself by inventing measurements nobody had really heard of. Some aren't so bad but others are garbage. Apparently people bought it, just like people bought into Florio's TMZ-style reporting. Now PFF is quoted on nationally broadcasted games and their, "information", is priced through the roof. Used to be like $30/year.
That site looks like it's a spinoff of other sites who are just trying to gain a foothold by throwing a bunch of numbers at the wall and seeing if anything sticks. It's kind of odd that it appears to be NFL-affiliated. I have no idea if they are, but they use the shield and their domains are pretty indicative of having some relationship of the league.
Hands down, the worst stat I have seen though. It says absolutely nothing.