You, sir, are were deluded and/or confused.
Inverse-square is relevant to 3-dimensional situations.

Try a little high-school calculus; Volume of a spherical wavefront increases with the radius cubed; how does the volume swept out by an expanding wavefront vary?

Nice to know that almost everyone has a certain transmission power; that tells nothing about receiver sensitivity.

To check the sensitivity by taking a walk down the street? You can do that, but if your street is typical, it won't tell you much. Multiple APs are likely to exist without sufficient frequency separation; you will then lose the weakest AP due to SNR, before the signal is too weak, so this sensitivity "measurement" is only helpful for similar circumstances; when you reject those other APs with a directional antenna, you'll need to re-analyze with the actual sensitivity.

But go ahead, and do experiment; the numbers would be helpful for use with an omni, anyway, and that's better than what you have now.

Edit: Some more concise people have beaten me; thanks, Brian and Mara.