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Reading and understanding your problem, the question was not the translation but a grammatical explanation of the use of the word "one" in the particular sentence.

Musk has played the troll so convincingly that it’s easy to forget he isn’t just *one*.

The word one can be used in 3 ways:

1. as a number: I have only one task to complete.

2. as a determiner: We will meet again one day.

3. as a pronoun (the plural of this pronoun is ones and the possessive is one’s): It’s a good cake, but her last one was a lot better.

In order to understand pronoun – antecedent agreement, you must first understand pronouns.

A pronoun is a word used to stand for (or take the place of) a noun.
A pronoun should refer clearly to one, clear, unmistakable noun coming before the pronoun. This noun is called the pronoun’s antecedent.

I allways wanted a doll but never had one.

Pronouns are used for referring to something when that (type of) thing has already been mentioned or when it is obvious what (type of) thing you are talking about. A pronoun is a word that stands in for a noun, often to avoid the need to repeat the same noun over and over. Like nouns, pronouns can refer to people, things, concepts, and places.

It was a problem, but not a major one.
I’m only using this radio until I can buy a better one.
Your experiences are ones that are shared by countless other parents.
the one/ones: I’m going to keep those boxes. The ones I want to get rid of are in the garage.
this one/that one/these ones/those ones: I’d never seen a game like that one.

Here is an excellent lesson on the pronouns created by Margaret L. Benner:
https://webapps.towson.edu/ows/proref.htm
https://webapps.towson.edu/ows/modulepaa.htm

https://www.scribbr.com/nouns-and-pronouns/pronouns/#determi...