I think your source is right that RD is homing in on taxing rental income.
When the new real-estate tax was rolled out a few years ago, people's primary residence were exempt from taxation, so people came forward in droves to claim residence.
RD can now focus on the remaining condos. Sure, some are just empty, used as occasional city dwellings, but many are probably rentals.
How will RD establish which are rentals? They don't have access to banking data, at least not yet.