Firstly, as has been repeated and seemingly ignored, he's saying he enjoys Dr Who and thinks it is a good show, but that it's slipping into the format of going:
1. Bad thing happens.
2. Dr Who does something without any explanation.
3. Everyone celebrates.
whereas it used to be a case of
2. Dr Who analyzes the problem and finds a solution that, while in the realms of fantasy, sounds like it would be vaguely feasible, and is based in the flow of the plot.
As for Pratchett not following his own rules, the Discworld is a magical world, and while it may not follow 'our' rules, it follows rules of logic, and to an extent, stereotypical narrative and the power of tropes. Such as that magic is generally more dangerous to use than NOT use, and that a million to one chance should work nine times out of ten.
As is shown by a scene where, while trying to fire an arrow into a dragon's weak spot, they feel it's not a million to one chance, so he's bound to miss at the vital moment, so he closes one eye and stands on one leg, along with other things, to make it hard enough to be a million to one shot, because "whoever heard of a eight hundred thousand to one shot working?"
It makes no sense here, but under Discworld rules, it explains why a single arrow could maybe take down a dragon.
Pratchett's saying that in the Dr Who world, unlike before, it seems like a dragon could appear over London, be invulnerable to all attacks from the army, and then at the critical moment of the episode, find a dragon slaying setting on his sonic screwdriver which dispatches the dragon and everyone is saved, as opposed to formulating a cunning plan like his previous incarnations would have.
while the Discworld has strange and magical things happening, I don't think it often takes the easy route in terms of plot devices.