However, you've forgotten something: those figures are from a standing start i.e. assuming no beneficial bacteria in the tank, which would be the case if it was a brand new set-up, of course.
Nope, there is bacteria taken from another tank or flower-bot for a seed. Atleast I think so, because in the text it is mentioned about taking "seed" from flower-pot (or another tank, but you can get some disease with).
For a start, if you were "seeding" one tank from another you would of course make sure the first tank was healthy. Wouldn't be much point otherwise.
Secondly, "seeding" a tank by using a flower-pot is nowhere near the same as "seeding" a tank by using half the gravel and an entire filter from another tank. In the first case, you have a few bacteria which need to colonise the whole tank and filter; in the second, you'd have millions of bacteria and the filter and half of the gravel would be colonised. Big difference.
"24h / bacteria" was only to showing how slowly nitrification bacteria increase their mass.
I'm aware of that, and its a very good point. However, if you've transfered a whole heap of bacteria it's not going to take that long to colonise your entire tank.
plus matured water perhaps
Don't take water from another tank; there is nothing useful, only bad bacteria and harmful compounds.
What is useful is that you have the correct pH, correct nitrate levels and whatever for your new tank. For instance, if you're setting up a hospital tank or breeding tank for a fish in your main tank, and you want to have exactly the same parameters. You take a matured filter from the main tank, some gravel or rocks (possibly) and some water. That way, when you move the fish it doesn't have the stress of adjusting to different water conditions.
I know it works because it's something I've done often.
they never actually need to do it again, provided their next tank isn't significantly larger.
Im not sure, because if you take gravel or filter from another tank you get also those bacteria which are in that tank. And sometimes those bacteria can be dangerous - like fish-tuberculous.
The beneficial bacteria should keep harmful bacteria at bay if the tank is well cared for and you simply wouldn't seed your new tank from an old one when there have been fish diseases. I'm not only speaking from theory, I've done this a few times and my local LFS does it all the time.