I believe you were either mistaught, or mislearned the rule.
*She is the Polish.
*John is the Dutch.
These sentences are both incorrect. The correct forms are as follows:
She is Polish.
John is Dutch.
The situation where you use "the" is when talking about general groups of people:
The Dutch live in the Netherlands.
And this is only in particular circumstances. What your rule seems to be getting at is that, with a word like "Dutch", you need the definite article in this context ("Dutch live in the Netherlands" doesn't sound natural). However, with a word like "Indians" that takes the plural suffix -(e)s, the definite article is not required for general statements:
Indians live in India.
It is possible to use "the Indians"; it just has a slightly different meaning from "Indians" without a definite article. (The version with a definite article has a slightly more absolute or collective sense, while the version without the definite article gives more of a sense of a generalization or stereotype that is thought to be usually true, but not necessarily true for every last Indian.)
In general, nationality words can be divided into several types.
Adjective and noun are the same form. For this type, the only way to talk about groups is by using the -(e)s plural, with or without a definite article "the". Examples: (the) Indians, (the) Americans, (the) Israelis, (the) Arabs, (the) Greeks. Common suffixes for words in this class: -(i)an and -i.
Noun with no suffix, derived adjective that ends in the suffix -ish. For this type, groups are usually talked about using the -(e)s plural of the noun, but (the) __ish may occasionally be heard or seen. Some might consider "the __ish" an error for words in this class, however. Examples:
- Pole (n.), Polish (adj.), (the) Poles or (rarely) the Polish (example)
- Dane (n.), Danish (adj.), (the) Danes, or (rarely) the Danish
- Finn (n.), Finnish (adj.), (the) Finns or (rarely) the Finnish
- Swede (n.), Swedish (adj.), (the) Swedes or (rarely) the Swedish
Adjective with the suffix "-ish", "-sh" or "ch"; noun formed from the adjective plus "-man" or "-woman". For this type, groups are talked about using "the __ish/sh/ch". Examples:
- Dutchman/woman (n.), Dutch (adj.), the Dutch
- Frenchman/woman (n.), French (adj.), the French
- Englishman/woman (n.), English (adj.), the English
- Welshman/woman (n.), Welsh (adj.), the Welsh
- Irishman/woman (n.), Irish (adj.), the Irish
Adjectives with the suffix -ese, and Swiss; these have no standard noun form.
For this type, groups are talked about using "the __ese". Examples:
- Swiss (adj.), the Swiss
- Portuguese (adj.), the Portuguese
- Chinese (adj.), the Chinese
- Japanese (adj.), the Japanese
- Vietnamese (adj.), the Vietnamese
...And there are even more patterns that I've probably forgot, like "Spaniard" (n.), "Spanish" (adj.), "the Spanish".
Also, see "Why can we say 'an American' but not 'a British'?" You might occasionally see an "-ese" word used as a noun (like "a Chinese"), although this seems awkward to native speakers.