Dr Who has always been free to ignore its own "laws" whenever it suits it. You don't honestly think that some vague reference to "only 12 regenerations" will cause the show to be cancelled, do you?
These things are put in for plot purposes - eg, at one stage, it was convenient, for the purposes of the plot, to state that weapons would not function inside the TARDIS. Goodness knows how often that's been ignored since, but it's quite a few.