Noble I don't think anyone was saying TT shouldn't be learned because it is tedious to do so. The disagreement between Pratchett and myself was merely if age determines tedium, and had nothing to do with the question if they should be learned at all.
IMO if they can be learned without tedium, that is better than if it does involve tedium, and in some cases this can be achieved. In various ways. Which should IMO be explored before going for the tedious route.
But if all else fails, then rote memorisation, however tedious it is, will be necessary.
Regarding the understanding multiplication vs TT memorisation: I agree with sirfred that knowing that 5x7=35, and even knowing that 35/7=5, does not help you solve the 4/7 of a class of 35 problem if you do not understand multiplication and division. If someone asked you 'what is 35/7?' and then 'what is 5x4? you would be able to answer, but if you didn't understand multiplication, you would never even know that that is the operation required in this scenario.
If you DO understand multiplication, knowing your TT will reduce the load on your working memory and so will make the question much more solvable. If you do understand multiplication and do not know your TT, the question is still solvable given enough time and perhaps pen & pencil, and a good working memory. But if you do not understand multiplication, the question is not solvable.