Quite so. I do believe you folks – that different North Americans diphthongize to varying degrees, and that Canadians tend to do it less than USonians – but this is just one of those things that most English speakers don’t notice about their own speech – until they’re forced to learn a language (such as Spanish, or many others) where you DON’T diphthongize at all (or hardly at all).
Comparable realizations occur when English speakers start to learn Hindi (you start to notice that sometimes we aspirate out p’s, and sometimes we don’t), or Malay/Indonesian (you start to notice how we palatize and lengthen consonants at the end of words), etc., etc.
Or, to turn it around, imagine a Chinese speaker learning to distingusigh between “r” and “l” in English. Same idea.
I’m American, I don’t say light as /ləɪt/, nor have I ever known anyone who does. That pronunciation would sound foreign to Americans, because that diphthong isn’t used in AmE. In fact, no allophone with the schwa in the diphthong is seen in any dialect of English, so it would sound especially foreign. The word is /laɪt/.
I’m British. And, my mum told me that f o r t y is the American way of spelling (fourty(The way she thought it was spelled.). But, she was wrong. I searched it up on Google. And apparently, f o u r t y is just an older word form of the number forty. And, forty is more of a modern word form. Man, the original spelling was more simplistic.