The article does have a section on exactly the issue (FOV) you note - it identifies that the 50mm lens is not the same with modern cameras (which have smaller sensors than 35mm film cameras), and recommends a 35mm lens for these cameras. Personally, I have a 35mm prime lens and quite like it, and I think it did help me improve my skills when I can't just zoom to get what I want in frame.
That's a different thing, which is about the "crop factor".
Modern APS-C digital cameras have smaller sesnors, with a 1.5 crop factor and will need a 35mm lens to get 50mm angle of view. This is not needed for modern full-frame digital cameras, which can use a 50mm to get 50mm angle of view just like old film cameras.
The article in that section just tells people to get a 35mm for modern (APS-C crop factor) cameras, because that's what gives the 50mm effective angle of view on those.
This issue is orthogonal to the parent's suggestion for 40mm.
To put it in different terms, the article in that section is concerned with "what lens you need on a modern crop factor camera to get 50mm effective angle of view - hence the suggestion for a 35mm physical lens).
Whereas the parent is concened with the actual effective angle of view you get, and suggests 40mm effective angle of view is better than 40mm effective angle of view.
To get such 40mm effective a.o.v, you need a 40mm lens on a film camera or a full-frame digital, and a 27mm lens on a APS-C digital (the kind of cameras the article has in mind when it says that "modern cameras have smaller lensors).
But I think we've all gotten a little lost in the weeds, as the original article was recommending it not specifically because of focal length, but also because of lens speed compared to the 18-80mm zoom lenses now common on DSLRs.
50mm on an APS-C is short-telephoto; frequently pleasing for portraits, but less-useful as a single prime. (I still recommend 50mm f/1.8s as a second or third lens for people using APS-C, but not as an only-lens.)
But it's also incredibly expensive. Fantastic glass though, I've rented it a couple times for some shoots
The second issue is that the normal lens focal length for a 35mm film frame, by the most common definition, is actually 43mm. There are, of course, other definitions. https://medium.com/ice-cream-geometry/what-is-a-normal-lens-... seems like a good discussion but I admit to having just skimmed it. So neither 35mm nor 50mm is particularly great. My X100V has a 35mmeq prime lens and I often find myself wishing it was a little narrower. (Though of course I might be saying the opposite if it were actually 50mmeq....)
If Fuji did make a 50mm X100, though, I'd be on the list. Ricoh made a great choice by bringing the GR IIIx to market. If I ever jump on the GR train, it'll likely be with that 40mm-equivalent model. Less versatile, but the images that do hit will resonate with me more.
Why would they? 35mm is much more versatile and better for family, travel, and street photography, which is what those cameras are used for.
My old X100 field-of-view is spectacular just the way the designer intended it to be.
https://shuttermuse.com/fuji-x100-conversion-lens-review-28m...
If Fuji were to come to market with a leaf-shuttered 35mm f/2 comparable in size to the X100's 23mm, I continue to maintain that there'd be a bunch of high-end buyers. It'd be the only 50mm-equivalent instrument of its kind on the market.
I personally use a duo of 28mm & 50mm. 28mm is great for street photography in city centers, and 50mm is great for slightly zooming in. I'm saving for a 40mm next. After that I'll ponder on a 20mm for landscape and ultra-wide things.
After getting used to sharpness of a good prime, there's no going back to a zoom.
On a mirrorless full-frame camera a 35/1.2 or 35/1.4 makes a fantastic, portable, all-around landscape and environmental portrait lens. Not so true for a 35/1.4 DSLR lens.