It’s TRUE!
Monospaced fonts are a thing for two reasons:
1. Historically, they were all computers did, because it is simpler to have fixed widths. (Remember Knuth's biggest contribution was typesetting -- it is complex). Do you know why the Macintosh changed this? Because Jobs was inspired by a calligraphy course he audited at college
2. Because lining up text is important for programming
Other than that, variable-width text is much easier to scan and read. Yes, even for people who spend a lot of time working with code.
You should also be setting a maximum line-width of about 40em, as it becomes hard to read text when the eye has so far to travel back to the left side.
You also shouldn't center paragraphs 99% of the time, because it’s better when your eye can 'seek' to a consistent left edge (assuming LTR languages!)
If you want to know more, please look it up or ask the nearest LLM!