Use of "the" with nationalities that have plural forms
I think part of the reason is that recently it has become more respectable to say "French people" vs. "The French". Similar to how it feels more respectful to say "A Jewish person" vs. "A Jew". I think in general, people are moving to "[Natn'l Adj] + people" which may have "the" as its article or the zero article. Some people may have told you to say "The [nationality]" to refer to all people of a country because of this, but that doesn't mean it is grammatically wrong.
But besides that, I would generally agree with the rule that you provide:
Rule 7.8: Use the definite article when referring to nationalities that do not have a plural form. Nationalities that end in -ss, -ch, -ese, or -sh have no plural form.
is correct. But one must take into account the other rules listed as well.
The American are famous for loving sugar
does not sound correct.
Of the options you give for replacement, almost all of them work to refer to a general characteristic of people from America, except the bolded one.
Americans / American people / the Americans / the American people / the people of America are famous for loving sugar
The usage of "The Americans" is where the "Extra!" rule takes over:
Extra! Use the definite article when referring to a specific group of people who have the same nationality.
So this is more likely to be applied to a specific (usually small) subset of Americans as opposed to all Americans. The most common example is when teams are competing on an international stage and representing their countries (eg. Olympics).
If, for example, the American bobsled team was famous for their love of chocolates and gummy bears, then an announcer could say:
The Americans are famous for loving sugar, especially chocolates and gummy bears. In fact, Americans - in general - love sugar.
In this example the announcer is first referring to the few people on the US bobsled team, but then expanding the statement to all Americans.
Trying to say the same thing for "French" or "Japanese" wouldn't be as clear so the announcer might use "these" instead of "the". (which could also be done in the above example)
These French are famous for loving baguettes. In fact, The French - in general - love baguettes.