Why utf16 in this particular piece of code for validating a link?

Question

I was looking for a way to validate that a string is in fact a URL and I quite liked this piece of code from post (3rd from top) How to check validity of URL in Swift?:

extension String {
    var isValidURL: Bool {
        let detector = try! NSDataDetector(types: NSTextCheckingResult.CheckingType.link.rawValue)
        if let match = detector.firstMatch(in: self, options: [], range: NSRange(location: 0, length: self.utf16.count)) {
            // it is a link, if the match covers the whole string
            return match.range.length == self.utf16.count
        } else {
            return false
        }
    }
}

My first question is: why did this developer choose self.utf16.count and not self.utf8.count or one of its other variations.

My second question is: How do you know this is actually checking that it's a valid URL? Does this meet these requirements? (The documentation for NSDataDetector.link is literally empty. + no comments in apple's code)

A URL is a valid URL if at least one of the following conditions holds:

The URL is a valid URI reference [RFC3986].

The URL is a valid IRI reference and it has no query component. [RFC3987]

The URL is a valid IRI reference and its query component contains no unescaped non-ASCII characters. [RFC3987]

The URL is a valid IRI reference and the character encoding of the URL's Document is UTF-8 or UTF-16. [RFC3987]

A string is a valid non-empty URL if it is a valid URL but it is not the empty string.

A string is a valid URL potentially surrounded by spaces if, after stripping leading and trailing whitespace from it, it is a valid URL.

A string is a valid non-empty URL potentially surrounded by spaces if, after stripping leading and trailing whitespace from it, it is a valid non-empty URL.

Any comments are appreciated.