Wow.

I actually read that whole PDF just now. It's kind of interesting.
Other than not explaining in technical terms, looking silly, and having
a lame brochure, it actually doesn't make any wild claims like I would
have expected.

It makes a claim of 3-5% reduction. It specifically says it doesn't
work for digital meters, and it is not a magnet, and it won't cause
inductive heating problems. It mentions a handfull of other caveats too
even. Since when does a scam make such modest claims and have so many
caveats? What is the world coming to? Will I start getting "Enlarge
your ***** by 3%!", or "Lose 1 to 3 lbs and keep it off as long as you
keep exercising and maintaining a healthy diet!" emails?

So can anyone answer the op's original question? Does a mechanical
meter overestimate when the load is imbalanced? And if so, can anyone
think of a plausible scenario, in any possible situation, real or
imaginary, that would let the device in the PDF picture (I can't tell
what it is -- just some kind of metal C-shaped thing I guess) do
anything that could even have a remotely possible chance of having even
a miniscule, undetectable, insignificant but still non-zero effect on
the meter? I.e., is this just a not-very-useful and very poorly
marketed invention, or a not very useful and poorly hyped scam?

-Kevin