Ontario Graduate Certificate usually requires the completion of a bachelor's degree whereas a diploma may not. I've not heard of a post graduate diploma being offered in Canada.
You need to check if the the college is a Designated Learning Institute.
https://www.canada.ca/en/immigration-refugees-citizenship/services/study-canada/study-permit/prepare/designated-learning-institutions-list.html
I was aware of the quality of education before enrolling for these Post Grad Certificates. Thinking back, I should have opted for a Master's instead of two Post Grad Certificates. They're essentially diploma mills to put it bluntly.
It's up to the border office to let you in or not irrespective of what is written online. You need to prove that you need to be physically present in Canada to complete your course such as lab or practical work.