I'm the one who posted on gradcafe. I'm not sure how they do the admissions, but they only gave me 2 weeks from the date of receipt of offer (27 feb) to accept or decline the offer. I guess from that tight deadline it means that it's probably rolling - they send out at most 30 offers in the first wave, and when people reject within the first 2 weeks, they send out the second wave of offers to top the number to 30 again, and then they repeat till they have 30 students.
Oh yea, and it's 30 because they mentioned it's 30 spots out of 340 applicants.
As for my priority, I applied because I was afraid I wouldn't get into any PhD programs and so I viewed it as definitely a backup. But after having gotten an offer and having researched a lot more, I'm actually quite excited to go there because I think it'll be a fun and rewarding experience. So I'm most probably going to defer my PhD for a year and do the masters. I'm not sure about the Canadian part of your question, sorry.