The page you linked does not clarify the possessive apostrophe rule for the pronoun "it". In the case of nouns like dog, or sword, or Justin, we get things like "the dog's collar is red", "the sword's edge is sharp", or "Justin's hat is cool". I think you might be applying that rule to where it should not. Possessive pronouns do not include apostrophes, which applies to "it", "her", "their"; instead, we get "its", "hers" and "theirs" respectively.Its actually both lol. Here: Here Are the Rules for Possessive Apostrophes if you can find where I'm wrong, and they're wrong, I'm completely open and would appreciate your insights. This is what I was taught in English, and if I've been doing it wrong, I'd be happy to know why if you don't mind.
On the same website, there is another article that goes into a bit more depth regarding possessive pronouns:
Source: 10 Apostrophe Rules Violations
In the case of definite pronouns like "it", the apostrophe is not used. For indefinite pronouns like "one", or "anyone", the apostrophe is used.
A bit of an aside, but I also noticed an error on the page you linked.
Typically, since "shoes" is plural, we use a corresponding plural pronoun like "these" instead. The correct sentence would be "these are Peter's shoes".
For future reference, I recommend using a different website to brush up on grammar. I can't speak for the whole website, but the page I've linked below is more concise and clear.
As their names imply, both possessive adjectives and possessive pronouns show ownership. The independent possessive pronouns are mine, ours, yours, his,…