Anonymous pronouns are just as valid as other pronouns.
You will let slip identifiable information on here over time, using pronouns does give some info away for free. Any info can help build a profile on you and if the above user had rarer pronouns that makes them even more identifiable.
If you have in comments or post told people you live in a certain town and that town has 10000 people, cutting the pool by 50% or more is bad.
Using their desired pronouns could also be a danger to them if government/family discover the account.
the better way to handle this is to intentionally lie periodically. if you want to do that with your pronouns, go for it. but for trans people, it's very difficult to completely lie about pronouns so we should lie about other things.
Shit, just fluently speaking any specific language with gendered pronouns probably narrows down the potential pool of people much further than a subjective estimation of one's gender that may or may not be a truthful indication of how one actually presents off the internet