You've selectively quoted to prove a non point. My house if I sold it could absolutely not be bought by e.g. my renters. My house would most likely be bought by someone who already owns a house and my renters would then be homeless, or would have to go back to renting 1 room for a family of 4 instead of a 4 bed house.
Not all renters can buy houses, so a reduction in rental stock absolutely drives up prices of rents as renters are fighting over the property (my current tenants offered over asking to secure the property but I told them no they can have it at asking).
If you think landlords selling up isn't causing issues with higher rents then I don't know what planet you're living on.
It's not "edge cases", most renters don't have a deposit or have other situations meaning they can't get a mortgage.