I have many events where people after 3.18 have been entering as long as they had a visa.
And can also face legal challenge when accused on improper grounds for denial. On the other hand, AIP really means nothing.
You are too naïve. Almost all colleges and universities are issuing letters of support suggesting mandatory requirement to enter Canada for students. How can agents tell which letters are genuine are which ones were made up just to let students in? After all, universities and colleges want to issue these so they don't lose tuition.
Disclaimer: These opinions are based on personal experience. I am not going to provide evidence for each claim but I really don't think any of this is far-fetched.