I suspect that due to variations in the earth's surface, and fluctuations in pressure due to things like air currents (and to a lesser extent perhaps even natural and man-made formations) the shock wave didn't travel uniformly in all directions, and wouldn't have been audible from the antipode.

Something that makes this extra intriguing: the exact antipode of Krakatoa seems to be in an inhabited area on the banks of the Rio Magdalena in Colombia.

I wonder if anyone was there at the time to notice a pressure wave coming from all directions.

The sound was only audible up to 3000 miles in radius, or about 1/13th the globe. It wasn't audible in the other side of the Earth (although it was detectable.)

Assuming it was powerful enough to get there, you would hear it.

At the antipode you would feel an identical pressure wave from both side. This would manifest as compression which you would feel since you are compressible.

A flag would not move in either direction (i.e. the wave is canceled out), but there would still be a compression wave in it.

Like imagine hitting a piece of wood with two hammers simultaneously.