This is safer with a Spyderhole (or flipper), but it does not prevent the handle from rotating back if the pocket edge pinches a dual thumb stud type knife, and something swipes the handle "open" through your pants... On the contrary, the spine against the pocket edge is exactly why a dual thumb stud will get pinched, because the fabric "converges" towards the pocket edge...
Tip down is more dangerous because a blade poking through pants is held rigidly after it has poked through... Tip up will tend not to poke through like this. A rigidly held "poking out" tip is pure murder, and the absolute worst case scenario...
Spyderco designs tend to have a broad portion of the blade sticking outside the handle when closed, so, as liner locks, anything hitting at this broad blade portion in the pocket could overcome the detent. The detent can always be defeated in any number of accidental ways: In Spydercos I would only carry lockbacks, and the Civilian is even safer with its tip turned down deep into the handle: I trust the Civilian more for that reason alone, and it is one of the reasons I carry nothing else without a pouch...
The OP's predicament happened to me with a CRKT Apache tip down in the back pocket, spine against pocket edge, dual stud held pinched by the pocket, the handle moving back, 1/2" of the blade poking out, slicing my finger deep enough to send me to the hospital: Never carried liner locks again.
Gaston