I disagree with this statement. What gets in the face of the trigger is going to depress the "tab" as well. I don't think it reduces the chance of an AD.
The armorer's manual clearly states that the trigger safety is designed to prevent the trigger from being operated by "inertial
or lateral pressure". Clearly that is two things it is designed to prevent, not just one.
Lateral pressure means that if the trigger is engaged from the side (as opposed to from the center) it can't be moved since it takes engaging the trigger directly from the front lower center portion of the trigger to operate it.
It can't prevent all snags, but it does make the trigger significantly more snag resistant.
If the gun is firmly pushed against an unyielding snag, or vice versa, the snag will fire the gun
if the snag engages the trigger safety.
However, the trigger safety occupies only the lower half of the center of the trigger. If the snag only catches the side of the trigger, or engages the upper part of the trigger the trigger will not move.
Furthermore, if the snag has some yield to it, even if it engages the trigger safety, experimentation will demonstrate that it will tend to ride up the curve of the trigger to the upper portion of the trigger where there is no trigger safety.
Finally, the trigger guard of the Glock is proportionally wider compared to the trigger width than the trigger guard of many of the other guns I've measured.
The combination of all those things makes the Glock trigger quite snag-resistant.
I did some testing with an unloaded Glock using a properly weighted magazine. I threw it into the air over a padded surface and caught it with a dowel through the trigger guard. I was unable to get the trigger to operate using that technique in spite of repeating the test until I was tired of throwing the pistol in the air. The dowel sometimes caught the trigger, but it always rode up the trigger to the portion where there was no trigger safety to engage.
What the trigger safety won't do is prevent the trigger from moving if it is pushed firmly into something that won't yield and that engages the trigger safety--for example trying to holster the gun with the finger still in the trigger guard.
I wouldn't make a general statement that a manual safety is unnecessary. Some designs do call for a manual safety, IMO. But I don't think that all handgun designs require manual safeties.