I think you have followed Alice.
The explanation you quoted uses a lot of words to say that when you cut an external thread you start with a diameter that is the same as the major diameter of the thread and cut in to the minor diameter of the thread. For in internal thread you need a hole with a diameter the same as the minor diameter of the thread and cut out to the major diameter. The depth of cut in each case, measured on the diameter is twice the thread depth. (And I used almost as many words.)
Of course the boss diameter can be a little less than the theoretical major diameter because a small flat is permissible on the crest of the thread. Similarly the hole diameter can be slightly larger than the minor diameter because a small flat is permissble. Actually if you are tapping, the tap drill size is quite a bit larger than the minor diameter because the torque requirements are too much to cut an absolutely full form thread.
If you are starting with a slightly undersize boss, or oversize hole the depth of cut is less than the theoretical thread depth.
An open mind is a virtue...so long as all the common sense has not leaked out.