No it's very likely in fact.

iPhone has been around for 15 years so there almost isn't a person on this planet who doesn't know (a) what it is, (b) what the competition is and (c) whether it's a right fit for them.

So if you're surveying existing users you're talking to people who knew before they purchased the iPhone that it was going to be a suitable fit for them. So of course the survey is going to be highly biased in favour of a positive rating.