Seems very reasonable to me. The advice is very dubious, but presented as absolute truth with no nuance. That’s bad, and, applying it more generally, if there’s one thing from the first row of tips that’s bad, it’s unlikely to be the only one.
This sort of thing reduces confidence. I can also say from looking through a number of others that they often lack nuance, and present a solution to a possible problem, without contemplating why that problem arose, and whether the solution is actually the right solution. (e.g. image distortion and background repeat are both a bit weird when you think about them, and the solutions offered are blunt and often not very good with no suggestion on how they could be improved. And flexbox wrapping basically assumes that horizontal scrolling is bad and needs to be fixed, which it isn’t always, but somehow also misattributes the scrolling in the diagram, making it seem as though it’s the flex container that’s scrolling, when in fact it’s just overflowing and causing its parent scroll area, probably the document scroll area, to have a scrollbar, and that is more likely to be problematic if not managed deliberately; but it also completely neglects to mention the more likely problem with flex overflow, which is your items not triggering overflow, but instead being squished narrower than intended, and that is the thing that would actually be worth while describing the solution for—flex-basis and flex-shrink.)
This sort of thing reduces confidence. I can also say from looking through a number of others that they often lack nuance, and present a solution to a possible problem, without contemplating why that problem arose, and whether the solution is actually the right solution. (e.g. image distortion and background repeat are both a bit weird when you think about them, and the solutions offered are blunt and often not very good with no suggestion on how they could be improved. And flexbox wrapping basically assumes that horizontal scrolling is bad and needs to be fixed, which it isn’t always, but somehow also misattributes the scrolling in the diagram, making it seem as though it’s the flex container that’s scrolling, when in fact it’s just overflowing and causing its parent scroll area, probably the document scroll area, to have a scrollbar, and that is more likely to be problematic if not managed deliberately; but it also completely neglects to mention the more likely problem with flex overflow, which is your items not triggering overflow, but instead being squished narrower than intended, and that is the thing that would actually be worth while describing the solution for—flex-basis and flex-shrink.)