They aren't saying landlords are the problem, they are saying Institutional Investors and Foreign Corporations being landlords are the problem.
Doubly so when they themselves lobby for policies that keep housing prices high, which is the primary thing preventing more people from owning rather than renting.
The problem with institutional investors is that in making it big business you drive up home prices which in turn drives up the rental prices. Since they can have a wide portfolio of homes, they are less likely to lower rent prices when demand drops, because they can afford to take the short term hit if it they think the market is going to pick back up again. AND they can play games like grabbing all available homes in an area in order to artificially inflate prices.