So even without trading a coin that is not allowed they can terminate or suspend your account cause they feel like.
But in any case, Huobi is referencing "high-risk investment(s)" which I usually assume to be margin trading with high leverage, etc., not privacy coins that don't introduce any more risk as far as trading goes as any other coin. In other words, while that statement is doubtless true, I don't think it covers specific coins and certainly not Monero.
OP, I know of Huobi by name only and have never used them, thus I don't have a bias in your case. I agree with examplens that more info is needed if there is any that would have influenced them to freeze your funds. I do wish you the best in getting them to un-freeze them, and whatever the outcome is please keep the community updated if you would. People like you speaking up when they're having trouble with exchanges helps a lot of people, whether you realize it or not.