The issue is he's using the term "back" as a player wearing an eligible number and referred to as a running back on the roster. Roster positions do not exist in the rule book though. All offensive players are defined as either a lineman or a back (unless they are in no-man's land). #68 is clearly a back if he's lined up behind the QB in front of the tailback. He's a lineman on the roster, but by rule on that play, he's a back. He's not eligible due to his number, but by definition he's a back. And the WR lined up on the line of scrimmage because he's breaking the waist of the snapper is by rule a lineman.
The other issue with the example cloudofdust is using is let's say the guard is breaking the waist of the snapper, he's a lineman. Then the tackle is breaking the waste of the guard but not the center, he's a lineman. Then the TE is breaking the waist of the T but not the snapper, he's also a lineman. If the LOS is the +20, you now have a TE lined up with his feet on the 16 but by his definition a legal formation. And the wideout on that side who needs to be a back is on or inside the 15. That's quite the V formation.