I've never understood the whole TKD <> HKD thing in some schools that will give Hapkido ranks, when the instructor appears to have made a shortcut to instructor level of HKD based on both being a Korean martial art.
Obviously there are well credited instructors who have learnt both styles but I've also read and talked to people who have rank in both with 'iffy' timelines as to how it came about, or have been awarded rank based on their TKD rank, which equally confuses me. There may be legitimate reasons, I just don't know what they are, or I am reading too much into it at a glance.
There are plenty of organizations that are happy to grow their numbers by bringing fairly plentiful TKD people in and transferring their TKD rank to Hapkido or whatever art it is they're promoting. Usually, there is a seminar attendance schedule that initially inducts them and then continues their acclimation of the new curriculum. There are also fees to be paid.
The logic is that a TKD instructor is already fluent in the terminology, has an understanding of Korean arts via their taekwondo base, and can either incorporate the new curriculum into an existing TKD class, or teach it as an adjunct to it.
Haedong Gumdo was fairly famous for this in its formation, though I have no idea if this is still a common practice.
So much of how well this works in practice depends greatly upon the background of the person in question. Relevant experience makes a huge difference, particularly in picking up the conceptual elements of the new art.
Obviously, instructors don't wish to advertise that they got their high dan in a new art by showing up to a few seminars (in the grand scheme of things, twenty seminars is only a few if you're getting a dan ranking of fourth or higher out of it), so of course their timeline is appropriately ambiguous; manufacturing TIG can backfire on you, especially if conversations go into any depth on the subject. At the same time, they don't want to tell you that they attended a seminar and got rank, so they give out iffy timelines and ambiguous training histories.
Funny, but I find that such people are often incredibly snobbish about rank. And if the new style is some kind of made up style or an offshoot of another style, they're even more snobbish. This serves to keep them on an unapproachable pedestal and thus prevent their rank from being questioned. It doesn't work when talking to people with any real experience, but they do it anyway.