I'm sorry, but hasn't he already applied for a postgraduate diploma after having a bachelor's degree currently? How does it sound stepping back from current qualification?
He's applied for a 2 year college diploma not a postgraduate diploma, as mentioned by him in his first post.
And even if he applied for a postgraduate diploma, it doesn't mean that IRCC will accept that it makes sense for him to incur the costs associated with studying in Canada for that kind of qualification, without the intention of remaining in Canada.
They have the historical data and trend analysis, they know people who have an undergraduate degree and are looking to do a lower level qualification in Canada, are primarily coming to the country to try to remain and don't have their studies as their main objective.
They do this in part to funnel people who want to live permanently in Canada towards applying for permanent residency, and also to dissuade the notion there is a backdoor option to coming to Canada, as opposed to making the significant, and in their view more legitimate undertaking, of studying for a undergraduate degree or Master's degree.