an hour

hi there

I read a post in EnglishClub and see whether you guys agree with him. Below is the explanation of 'an hour'?:

The "correct" form is:
one hour and a half // an hour and a half
a mile and a half
one ounce and a quarter

However, in normal English usage, native speakers often say:
one and a half hours
one and a half miles
one and a quarter ounces

simon 

Agreed.

How can they say that a form that millions of native speakers prefer is incorrect?

I think most people would use the second versions. I do know some people who are not native speakers who say 'one hour' rather than 'an hour'.