Not quite true, because all the dibber actually does is record the time at which it is placed into the SI box, so for example, you start at 10am, you dib at CP1 at 10.13, which is recorded, CP 2 at 10.45, which is recorded, finish at 11am, which is again recorded. You then go to download and the software works out that it took you 13mins to 1, 32 mins to 2 and 15mins to finish, and 1hr overall.
Now, at most O events, you "punch" at the start Kite - i.e. dib the box at the start and head off. For races it's slightly different as you don't have a punching start and instead, I believe a common start time is input into the software and the race time calculated from that (this is also done at large O events like the british champs, where everyone has their own specific start time, but the point is it that doesn't have to be encoded on the dibber).
So, you could have collected your SI card from registration, but you the system would have shown you as having started. To avoid this, you would have had to report to download, so they could put you down as a DNF, but you could have done this within seconds of the racing having started if you wished, and you'd have then be counted in.
But your original point is still valid, because provided you downloaded, you'd have been counted out and then counted back in.

