Why does copper pour cause impedance discontinuities?
A track surrounded by a copper pour makes a Coplanar Waveguide:

https://en.wikipedia.org/wiki/Coplanar_waveguide

This is a form of transmission line, and you can make it with a controlled constant impedance provided there is enough copper on both sides. As with any transmission line, current traveling in the center conductor is mirrored by its return current which travels in the two copper "sides" sitting next to it.

Any interruption in the copper "sides" breaks the transmission line and creates an impedance discontinuity.

Component-side copper pours tend to be discontinuous and interrupted in many places by traces. They quickly turn into a mess of bits of copper strewn all over the place. In this case, your traces are made up of bits of coplanar waveguides with the side grounds haphazardly connected to a ground node somewhere, sometimes by very long bits of copper.

The top copper pour will also allow signals from one trace to couple into nearby traces.

A ground plane placed below is a much better option if you can have it.