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FCC Public Notice 14089 dated July 24, 1991 is available through the link in the OP, and appears to contain information that contradicts some of the applicable paragraphs of Part 15. But in any case (IMO) it would/should not have the force of law. It is an advisory notice.
The actual law applying to unlicensed use of the AM/FM broadcast bands is given in FCC paragraphs 15.209, 15.219 and 15.239.
For example, the public notice states that the maximum radiated power for AM is 0.05 watts (50 mW). However that amount of radiated power produces a groundwave field intensity that is far above the nominal 15 µV/m value permitted by §15.209 for the upper end of the AM broadcast band.
The radiated power needed to just meet §15.209 at the upper end of the AM broadcast band is on the order of 2 nW (0.000 000 002 watts). Radiating 50 mW would produce a field at 30 meters that is over 4900 times greater than the ~15 µV/m value meeting §15.209.
FCC §15.219 permits 100 mW (d-c) input power to the final r-f stage, and limits the radiating length of the antenna conductors to 3 meters, including the conducting path to r-f ground.
Probably the intention of the FCC was to provide an alternate way to comply with §15.209 without the need for accurately measuring the fields radiated by these systems — which probably very few hobbyists are equipped to do.
But in reality, a system meeting §15.219 can produce much higher radiated fields at a distance of 30 meters than is permitted by §15.209. So these systems, while they might technically be compliant to a given set of rules (209 or 219), that doesn’t necessarily mean that they will have the same radius to a given, groundwave field intensity.
(Written before reading Neil’s statement above, but really there is no conflict between our two posts.)