I guess you are right. The only explanation I have at this time is partly the recent hibor rise and partly that IB's trading desk has hkd demand, though the latter is a rather poor explanation.
I checked and all other hkd rates I got are pretty much in agreement with us dollar rates, without surprise. The IB one is the one that is completely off and upon checking with IB they stubbornly insist their rate is correct. Well, I converted my idle usd balance to hkd a short while ago and enjoy the ride while it lasts.
They have a large HK based and registered entity. Can't say what percentage of volume goes through the HK entity vs the US one, and for any funding purpose they certainly access markets any time themselves.
It's showing 1.933% now, pretty much in line with interest rate on USD idle cash https://www.interactivebrokers.co.uk/en/index.php?f=39726&p=schedule I don't know what the fuss is all about. It might be a mistake that IB made on its website and some noob IB CS who just doesn't know what he/she is talking about. Both very possible scenarios considering it's IB.
The only high interest that's being paid on idle cash now is actually on MXN, RUB and ZAR. So maybe you should consider converting your idle cash to one of or all of those currencies considering you have 8 million USD (I assume) of idle cash.
The EM ccy rates you mentioned are all fairly priced. It is the hkd rate that was mispriced on IBs website. They apparently corrected that now as it showed 4.xx% until a few hours ago. Strange that two reps insisted it was the correct rate, one from the trading desk and the other in funds and banking. Perhaps it was me who alerted them to this issue because IB in fact paid out 3.55% on the hkd cash that was sitting in my account for the past days (albeit just a fraction of my entire account balance). I verified that just now via my daily activity statements from the past few days.
They are noobs. You shouldn't have called them to verify. Might have allowed you to enjoy the high rate for even longer time.