Well, it's not exactly the tackle box. The tackle box as defined (2-34) may extend past the 2nd lineman from the snapper, whereas the area in 2-b, only extends laterally to the outside of the 2nd lineman from the snapper or 5 yards whichever is shorter.
The area in 2-b may be as wide as the tackle box, but it could be "thinner" if the outside of the body of the 2nd lineman is less than 5 yards from the snapper.
I actually am glad that it is written this way, because it gives us a benchmark for the position of the back that we can actually see on the field. We no longer need to visualize the tackle box and officiate "air". Unless the line has incredibly wide splits (which is to say, almost never), we can use the tackle as that benchmark (officially).
2-b defines two areas: one bounded by the outside edge of the tackles if the edge is within 5 yards from the snap, and one bounded by the tackle box. It was intended to make it clear that any player who is outside either of those areas is restricted.
Yes, a very good change that takes us back to where we were a couple of years ago in determining where a player is positioned at the snap. As for determining where a block takes place during the play, using the edge of the tackles may help or it may not depending on how the play develops.
Now moving on to "3." It tells us that a player who is restricted at the snap may never block low inside an area defined by 2-b. Since one of the areas defined by 2-b is the tackle box, then that area seems to be, by definition, included in the "no BBW area" for a restricted player and since that area is always the outer limit of 2-b, then I am certain we can say, 2-b = tackle box for purposes of "3.".
In simpler terms:
To determine a restricted player at the snap use the edge of the tackle if it is inside the tackle box. Otherwise, use the tackle box.
After the snap, use the tackle box to determine where a restricted player may never block low.