I know not all schools that teach Hapkido will do this but I once knew of a school that taught both Tae Kwon Do and Hapkido. In order to train in Hapkido at that school first you had to get a black belt in Tae Kwon Do there. I do know that Hapkido does use some of the techniques from Tae Kwon Do but to even start as a white belt in Hapkido at that school you first had to get a black belt in their Tae Kwon Do. I wonder if other schools have that same requirement.
I teach hapkido and have no requirement that you learn taekwondo first. Honestly, that makes very little sense, unless his path to black belt in taekwondo is ridiculously short (one year at most), and even then it still makes very little sense.
There is a perception amongst some that hapkido is a more advanced art than taekwondo, thus learn the simpler to get the more complex. I don't subscribe to that notion, as taekwondo is every bit as advanced as hapkido.
The two arts are similar enough, and as others have mentioned, hapkido grappling techniques are frequently grafted onto taekwondo classes. I studied in such a class for several years, and loved it. But the class was structured so as to add the various grappling techniques in a logical progression that matched the taekwondo belt progression rather than being an entirely new class that you took after getting your black belt in the base class.
After your taekwondo black belt, you should be looking to deepen your knowledge in taekwondo, not just jump to another art that's similar but with added grapples.
The closest thing that I've seen to that in my area was a Jinenkan school that reserved the taijutsu classes for adults (ages 16 and up). The childrens class was (according to them) based in Isshin Ryu karate. The junior class was the same class, but with some taijutsu elements (mainly escapes, sweeps, rudimentary throws, rolls, and falls from what I could tell). Students who stayed in the school long enough to transition to adult classes transitioned to the taijutsu class, though so far as I know, there was no requirement to earn a black belt in the junior or children's class.
Aside from the suggestions put forth by others here, I can think of no logical reason for a school doing what you are describing, and being very familiar with both hapkido and taekwondo, I would consider it to be counterproductive; there is so much overlap in techniques between the two arts with regards to striking that there is really no reason to learn both.
Both arts are fairly 'complete' on their own (meaning that they are complete systems, not that they cover each and every range/combat possibility, not subsets of a larger art), and continued study and depth in either one after earning that black piece of cloth is probably more valuable than jumping ship and going to another art unless you've
really determined that the first one wasn't for you and have had enough exposure to the second to know that that's where you want to go.
I had studied TKD for many years before taking up hapkido, and I mainly did so as a favor to my kwanjang, who was trying to get a new class off the ground. By the time I had my black belt in hapkido, I was fairly sure that it was a better fit for me than Kukki taekwondo was, though I did continue to stay involved in taekwondo for awhile afterward.
I won't tell you to avoid the school (given that you seem to want to go directly into HKD, you're probably inclined to look elsewhere anyway); I've never visited them and won't make an evaluation of them on the internet. I will say that I consider what you describe to be an incredibly inefficient way to get into a hapkido class.