Looking at my copy of a 560 AFM, there is no reference to DH anywhere, but lots to PH. Certification work is based on standard conditions and then applies deltas for off standard. At standard conditions, PH and DH are the same. It would make little/no sense to presume that height references are other than to PH UNLESS the AFM specifically noted the difference - I can't recall ever having seen such a situation in respect of limitations.
Having said that, there would be no operational constraint against a crew's deciding to fly lower if the OAT is above standard - that's in the realm of operational decision making. At all times, there is only one really hard and fast rule - "don't crash" - all the rest of the rules is legalese window dressing which one is well-advised to observe.