What is...I don't...
Isn't the point of a handwave to sidestep engaging something because it doesn't make sense?
(Personal opinion--the above aside, I don't believe it's a hand wave. It's just one of several time-travel-related solutions/outcomes. Where it falls on the DEM/Brilliance! continuum depends on personal preference, but it's a solution not only established in time-travel fiction (see: Bill and Ted for one), but in Who as well. After all, just look at the end of Blink.)