Is the striker block safety in the correct vertical position when the slide is off the frame? I don't know.
It seems possible that the design requires the block to be not at the bottom, not at the top, but somewhere in the middle to block the striker. If that's so then having the slide off the frame may allow the block to be below the range where it can stop forward striker movement. And that is not a test of the safety as the pistol would not be carried/fired that way.
I don't have a P10, so I don't have one to "play with."