There is a huge difference between small and large landlords. The example I gave was clearly related to small landlords. If you have two houses and two mortgages and are doing your own maintenance, you aren't driving up the cost of housing significantly. If you are a small landlord, as I've described, the only "profit" you're making goes straight into the payments on that mortgage, most of which is interest for the bank. Also, those "profits" won't be realized for 20+ years. Of course, I'm talking about averages over time. Clearly, housing is unbalanced right now, and bubbles create exceptions.
Large landlords, hedge fund investors, foreign investors, large AirBnB investors... these are a different story. They are the ones on large amounts of property, creating artificial scarcity, jacking up rents to unreasonable levels, etc.