In order for the P365 to fire, the striker safety (sear block) must be pressed inward. Pulling the trigger not only actuates the sear, but it also actuates a lever that presses the striker safety inward just before the sear releases. In order for the P365 to fire WITHOUT pulling the trigger, the striker safety must be stuck in the inward position. Then there still needs to be some force, such as dropping the gun or the sear spring breaking, to cause the sear to move and disengage from the striker.
If the P365 dropped upside down onto the slide hard enough for inertia to move the striker safety inward, the inertia would likely also be pressing the sear into the striker making it very unlikely for the sear to release from the striker.
I did read about someone that had the striker safety jam in the inward position. He used a solvent or penetrating oil to free the striker safety.
Every time I have the slide removed I check the striker safety to make sure that it is moving freely. So far the striker safety has always moved freely. If your P365 ever got wet you should remove the striker, striker safety, and also the extractor, and clean and dry everything ASAP.
Even if the story is true, having had a manual trigger safety engaged would likely have prevented any possible uncommanded or negligent discharge. The manual safety on a Sig P365 prevents the sear from moving far enough to release the striker.