HODLing it in custodial exchange or own wallet wouldn't change the fact that they hold their investment through time and didn't sell it.
So I would consider them as HODLER's the definition of a word doesn't change no matter what they use to keep their investmenr.
In fact, only those who plan to hold for the long term will be considered as holders. Considering that aspect, those who hold in the exchange wallet cannot be called holders in any way because they do not own the wallet, but rather they have stored their assets with a third party. However, to become a holder, you must ensure that you have access to your own wallet and that no one else can access this cryptocurrency.