It is a 1-D measurement. It's the pressure exerted by the atmosphere that would raise a column of mercury that 1-D length up a vacuum at 1g of gravity.

It doesn't matter that you're measuring it in an odd way - pressure still has units of force per unit area. 1 "inch Hg" is 3386 Nm⁻².

The fact that you can express length as "number of football fields" doesn't make length dimensionless.

Even if not taken literally, the particular unit of "inches" is largly meaningless in this case. All we know (assuming the obvious mechanism of measurement) is that the air pressure in <choice of force units> is linear to the reported value. The conversion factor is dependent on the instrument itself (surface area, internal resistance, etc) although there may be a standard that is known specialists.

Inches of mercury -- the pressure exerted by a column of liquid mercury that high.

Two square inches of mercury an inch tall weights twice as much as one square inch of mercury an inch tall. But there's twice as much area, so it's applying the same amount of force to each square inch.

The point is you care about the force, but don't care about the area, and a tube of mercury factors out the area.