The MDI defines "owner occupant" over "landlord" in such a way as to effectively subsidize the single family detached suburban home lifestyle preference. It encourages the middle class to tie up a big chunk of their net worth in sprawling developments with questionable claims of being an appreciating asset.

I’m trying to gain understanding of your first sentence, but have been unable.

Landlords can deduct mortgage interest just like any profit-seeking business can deduct interest on loans taken on in furtherance of their business. That’s true for single-family homes or duplexes in a suburb same as a rowhouse, 4-plex or 50-unit building in a city.

The owner-occupant deduction applies to their unit, whether that’s all of an SFR, half a duplex, one level of a triple-decker, or 1 unit in a 4-plex.