I'm not surprised that a lot of the smaller exchanges are doing this. Perhaps erroneous to call them HYIPs, but I would fully expect a lot of exchanges to use their substantial holdings for low-risk ventures. Those multiple alt air drops based on Bitcoin balances, low-interest services (like freebitco.in), intra-exchange loaning, for example. Some exchanges' wallets are known, others may be easily discovered. Pretty sure some of these wallets from OK and Huobi were linked to investment addresses.
Difficult to argue about what constitutes HYIP if this is a legal pursuit but OK's response indicates that they have been transparent about investing inactive funds. If private equity can't be forced to reveal their links, I wonder if the same confidentiality applies here.
I doubt that we will ever learn the truth about the use of funds. While there is no evidence, but there are just mutual accusations and speculation.