I can't speak for the author making the claim, but I would imagine that they're not saying absolute demand for condos is weak (which would result in unsold inventory, which we clearly have not seen), but rather RELATIVE demand is weaker compared to the market for detached houses.

However, comparing the graphs in price increases since 2001 (see here: bit.ly/2qcrzaJ )… I can't see any clear evidence for that claim. Median prices of detached houses appear to be ~1.7x higher than condo prices in 2001, and while both markets grew substantially, the ratio of prices looks about the same in 2013.

But since the data is 4 years old, it's possible that things have changed since then, and detached house prices have really taken off.