Having cycled in that pedestrian zone several months ago when I was in Brussels, I can certainly see why they would want to ban cycling in the area.
I got the impression that it was too busy with pedestrians to meaningfully cycle there. It would be better if cyclists dismount and walk if they need to be there, and otherwise cycle around the area.
The alternative suggestion in the article to clearly mark a bike lane with paint, would only work if the pedestrians also honour the paint by not walking in the bikelane. Somehow I doubt that that will happen.
Sometimes cycling and walking are incompatible, especially if there is large masses of people. If you can’t have both keeping the space as pedestrian-only seems like the preferable outcome.

On the website of the municipality there is a list of all streets that will have their maximum speed lowered:
https://www.utrecht.nl/wonen-en-leven/verkeer/verkeersveiligheid/wat-doet-de-gemeente/straten-30-kilometer-per-uur-maken
Sadly they don’t show a map. That would be a nice visualization.