I'm confused by this statement. In any older city in the US, like in any city in Europe, the heart of the city is apartment buildings. Those would have been the main part of the city by the time it was big enough to be called a "city." Single family homes were generally in later areas -- suburbs, or suburbs that got swallowed up by the city -- or in the small exclusive pockets of extremely wealthy areas.
As for Amsterdam - it's a tiny city area wise, 30 km/h is mostly core. All highway limits stays the same. Map they provided show 50 km/h outside of the very centre. I'm glad for them if that make a difference. As a side note I didn't enjoy Amsterdam public transport (again comparing with Montreal).
As a related fact in Montreal that stretch of the road https://maps.app.goo.gl/dgG8avYABUgbZ7BTA was switched from 50km/h to 40km/h. As you can see it has fences on both sides, so it's hard to justify safety reasons here.