It is true because all you are collecting there is what your visitor has their browser set to identify itself as.
Opera, Chrome, and Safari can all identify themselves as Firefox or Internet Explorer.
Internet explorer and Firefox can identify themselves as anything at all.
If the browser claims to be "Eat at Joe's" version 21 how are you going to tell if it is really IE 7 or Firefox 9 or some other version of one of those browsers?
If the browser identifies itself as Internet Explorer 27 how are you going to tell what browser iand version it really is?
All you can collect with the code you are referring to is what browser I decide to tell you I am using - and if you use that antiquated code (obsolete even in Netscape 2) then likely your site will not work propelry for many visitors and they will send you a fake value just to bypass your code. I have to do that to access the web site for my bank because they stupidly check for certain valuees in those fields even though their site works in other modern browsers as well.
Fortunately most modern browsers can be set to provide different information to different sites based on what browser you want the site to think you are using.