Here is my limited experience:

A bike that feels more "rigid" on the ride doesn't necessarily mean it's stronger. It means that when you ride with it feels more stable and less vibrational on the road.

For example you'll feel like that the bike is set on a track on a road on which it smoothly and predictbly moves forward. While a less rigid bike will feels more jittery.

Hard to describe. Gotta experience it yourself.