This is because it has something to do with the overall difficulty (OD) in osu!. Such as OD2, for example, which has 68ms of timing window to SS. With DT, the timing window becomes 1.5x stricter, which in fact currently is 45.33ms (OD5.78), that means it's harder to SS. With HR, the OD becomes 2.8, and with DTHR, this makes the strictest timing, which in this case, OD2 becomes OD6.31.
Formula for Perfect (300): 80 - 6*OD
You can set the overall difficulty to only 10, which is the most punishing timing window (20ms for 300), but with DT, it's even more insane, and breaking the limits.
mrekk's Team Magma SS play on Catastrophe +HDDTHR should be respectable, as OD11.11 is not that easy to SS. With 13.33ms of timing window, he's able to SS, as either he is lucky or his skill is unfathomable. Yes, mrekk is the #1 player in osu!
For any maps that have OD7.2+, you can use DTHR. For any maps that have OD10, you can directly use DT.
I'm sorry if this text is just a nonsense, or me being sarcastic, but just some information, and my experience to SSing maps with (HD)DTHR (or HDDT, or even DT only)