The tackle box does not move - it is the area between where the tackles started.
It really isn't all that complicated. If the QB is in the tackle box, he has to throw it to a receiver. If he leaves the tackle box by running to one side or the other, he can can still throw it away legally by throwing it to a receiver, but he has an additional option of throwing it away so that it crosses the line of scrimmage. In this case, I think TT was outside the tackle box, since he was on the other hashmark, so he had two ways it would not have been grounding - if it had been close to a receiver, or, if it had crossed the line of scrimmage. Dovell was coming back, but didn't get close enough to it, and it didn't make it to the line of scrimmage. Thus it didn't meet either exception.
Billy actually stated it right in his first post:
... In fact the penalty of grounding is thrown by the R after conferences with the LJ (in this situation) that there were no recievers in the area or in this case an attempt to get rid of the ball did not reach the LOS. ...
but then muddies the water by leaving out the option of it being close to a receiver:
...The pass has to make it back to the LOS, the pass did not, so it's Intentional Grounding.
and then says TT wasn't outside the tackle box, which I believe he was:
...the QB was NOT outside the Tackle Box, and the intent of the throw was to avoid the sack that is grounding, the MAC office confirms this was a correct call, the spot was the error here.
Note - if Billy is right that TT wasn't outside the tackle box, it wouldn't have mattered if it crossed the line of scrimmage - it would have had to be close to a receiver. In his earlier posts, Billy implied that it did matter if it crossed the LOS, so those posts are inconsistent with this one.
Then, to muddy the water further, after posting a correct version of the rule:
..The intentional grounding rule as written states: It is not a foul when the passer, who is or has been outside the tackle box, throws the ball so that it crosses or lands beyond the neutral zone or neutral zone extended.
in the same post reverts back to an incorrect version:
If you are going to rid yourself of lost yardage, there are certain things that must be meet, and throwing the ball to or beyond the neutral zone is one of those while inside the tackle box is one of those, unless there was an eligible reciever in the area.
If TT is still inside the tackle box, throwing to or beyond the neutral zone is not sufficient - it must be to a receiver. If he is outside the tackle box, it can be either to a receiver, or it can cross the LOS.