I'm puzzled by this claim. I wouldn't call it a myth, but all the same...

I grew up on a vineyard. Each harvest time the grapes would hang down heavily and almost touch the ground. Our pet dogs would park themselves beneath and greedily consume a whole bunch.

They would also scoff any dried grapes they found on the ground.

The worst effect seemed that they all would get noticeably pudgy during the season.

The concentration of tartaric acid in each individual grape is unpredictable, along with each individual dog's tolerance level to tartaric acid. The only thing that's certain is that anything with tartaric acid is potentially toxic to canines due the development kidney damage after ingestion.

AM is a type of modulation which is widely used by many different services (eg Shortwave broadcasting, CB radio, Aircraft VHF, etc).

However "Longwave and Mediumwave Broadcast" unambiguously refers to that service.

Transmitters need only be re-certified if they are to be sold in quantity.

Once-off Broadcast Transmitters need only demonstrate that they meet the relevant standards.

Many of the existing LF and MF BC transmitters were effectively custom built to meet the requirements of their transmit license (for example, the radiation pattern of the antenna were often unique for each station).

It doesn't. In both it depends on what the stations engineer chooses to transmit.

Most cheap FM receivers definitely use a PLL to detect FM. Giving automatic tuning is just a side benifit.

Do you mean AFT (automatic fine tuning)? This has nothing to do with a PLL-based demodulator. Older receivers with ratio detectors or discriminators also had AFT- there is a low frequency feedback path from the demodulator to the local oscillator to make this work.

Most FM receivers can lock on the carrier because they have a "Phase locked loop" to cancel any tuning errors.

Many good AM receivers do exactly the same thing, especially those receivers which have "Synchronous Detectors" for AM.

It's just that the circuitry involved is simple for FM, but rather more complex for AM.

Is commonly known as "Bass Boost".

As the OP has said, it cannot give louder bass, but simulates the bass harmonics.

This is yet another myth:

The "Woolyness" of AM broadcast (at least in America) is due to the stations purposefully tailoring their audio processing to suit typical cheap AM receivers. And this in turn is because designers of cheap AM receivers fit narrow filters instead of using noise reduction techniques, eg a good outside antenna.

There was a period (in the rest of the world) where high quality AM receivers had a narrow/wide switch to give better audio response to stronger signals.

The good news is that modern SDR receivers usually have selectable bandwidth on AM so as to derive the full transmitted audio. And many of these have AM stereo decoders as well.

If you listen to a good quality AM broadcast (eg Gov AM stations in Australia) you will hear audio which are very hard to tell from FM audio.

Go back and read the many high-quality AM tuner articles in the electronic hobby magazines from the past.

Yes, when I was a kid I used to listen to a few AM radios in a Marantz receiver from my dad (not in the US). Well, it was not as good as FM, but basically it was usable as a source of music.

Now, from time to time I buy a cheap portable AM radio, mostly out of nostalgia, but with the excuse of being good emergency preparedness, and the sound is annoyingly bad even with decent headphones.

Only while signals are strong. On weak signals however, AM has a considerable benefit in intelligibility over FM.

FM stays good as the signal weakens, and then kind of drops off a cliff almost.

This is a myth. There is no reason that channel spacing need limit the modulation bandwidth. The only downside is that listeners to adjacent stations will hear a slight "monkey chatter" from the overlapping sidebands. In reality stations are never allocated adjacent frequencies within the same coverage area so this usually doesn't happen.

Be that as it may, AM radio is obviously low-pass filtered. It might not be a brick wall at 5 kHz, but it sounds obviously muffled to someone who can't hear anywhere near up to 20 kHz. If I were to guess, based on years of experience of playing with EQs, I would say that it has next to no content beyond somewhere around 8 kHz.

AM receivers have to apply a band pass filter to select the station. The width of that filter will directly affect the audio bandwidth. The filter probably can't be anywhere near +/- 20 kHz based on the idea that nobody nowhere would put two stations close together in the same area. Mass produced AM radios have to work everywhere.