Different light sources emit with different "Coherence length". A continuous-wave laser has high coherence length, and a light source that isn't a laser at all

Different light sources emit with different "Coherence length". A continuous-wave laser has high coherence length, and a light source that isn't a laser at all has low coherence length even if it's monochromatic, for example a sodium lamp. The laser used by the presenter has a coherence length longer than (or in the same ballpark as) the difference in optical paths in their experiement, so they get a clear interference pattern. The Wikipedia article may explain. https://en.wikipedia.org/wiki/Coherence_length https://en.wikipedia.org/wiki/Coherence_length Since you can measure coherence length (and higher-order temporal and spatial coherence statistics), it is part of the information carried by light from a luminous object that is available for imaging by a suitably designed camera.