It's quite common to find these topology errors when dissolving data:
Census tracts dissolved by neighborhood using ST_Union (PostGIS)
It's quite simple to understand the concept of a polygon donut hole in these cases: when the geometry really had a gap. But when your resulting holes are lines or points, my logical thinking get stuck.
What exactly makes this happen or not? If the polygons share the same boundary lines without gaps, how does it create a line hole? A point feature within this 'line hole' wouldn't intersect with the polygon feature?
I'm looking for a more detailed explanation, rather than the general and common "This happens when working with bad quality data"
Answer
Your logical thinking is fine: those are polygonal topological gaps in your original polygon layer, but the gaps are so small (or narrow) that they appear visually as lines or points. If you examine the geometry, either by going through the XY coordinates of the vertices of the gaps and/or by zooming in really close, you should see that they are all polygonal. As you say, if they were actually points or lines, they would be 0- or 1-dimensional features and wouldn't constitute an actual gap in the polygon layer.
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