This short opportunity is based in fundamentals that the Canadian Real Estate Market is overvalued and house prices are already in the correction territory in cities such as Vancouver and Toronto.
Considering that REITs seems not be affected this year so far I believe that is about to change. It might be a just a small pull back and the prices can still be sustained for at least a year. However, the main principle is that upside is bigger than the downside if you are short Real Estate. In addition, REITs are risk averse for the most part, so the profit/loss exposure can be reduced.
Anyway, in addition if you look at Housing Bubble in US, one of the first signs of bursting it was the REIT falling apart before anything else.