A query about the use of Generic Reference with nationalities

Your belief is incorrect. Both of the following are acceptable and effectively synonymous:

1. The Italians are musical.
2. Italians are musical.

Furthermore, #2 is actually more common than #1 and seems more natural to me.

There are three different types of generic Noun Phrase, distinguished by their articles.
They are similar in meaning -- positing some proposition over a set -- but they are different.

Plural Generic NPs use no article.

• Pandas are found in China.

Definite Generic NPs use a definite article with a singular noun.

• The panda is found in China.

Indefinite Generic NPs use an indefinite article with a singular noun.

• *A panda is found in China.

Note that the last sentence is ungrammatical, while these three are all fine:

• Pandas eat bamboo.
• The panda eats bamboo.
• A panda eats bamboo.

As for national or cultural names, these vary with the type of generalization being expressed.
And also with the socioeconomic status, and general prejudice against, such groups.

On the whole, generic expressions about human groups are best avoided;
they are far more apt to shed heat than light.

For what it's worth, in my American dialect, I would never use "An Italian" to mean an Italian person. I know you do that in other languages, like French, but I wouldn't say that in English.

Frankly, I wouldn't say "A cat is cute" either. To me, that construction suggests that every single possible cat is cute, which isn't true. "Cats are cute" only suggests that most cats are cute. at least that's how I would use that construction.

I might say "A square is a rhombus", because that is literally true for every single square.