I'm not sure either, but I think the question could be rephrased as:
Can the column boundary marker actually enforce the column limit, instead of just visually indicating where a break should ideally occur?
So, imagine the marker had the same effect as manually narrowing the main window's right border, e.g.
<img alt="geany-hard-break-at-visual-line-break-col" src="https://github.com/geany/geany/assets/59004801/57ef738c-7612-4d72-bddc-7d9d150e3a25">