I'll let Curvy Kate have the last word...
After all they make, fit and sell bras FOR A LIVING!
https://www.curvykate.com/blogs/news/want-to-know-the-real-reason-why-women-are-wearing-the-wrong-bra-size#:~:text=The%20'Plus%204'%20Fitting%20method,from%20the%20difference%20in%20measurements.%22&text=Today%20specialist%20bra%20fitters%20(that's,don't%20'measure'.
Well firt off Curvy Kate is a UK bra maker so surprise surprise surprise they would sign on to using this UK methods
I do not know how long they have been in the game but I am fairly certain that the likes of Playtex, Frederick's of Hollywood, Madenform and Victoria Secret have been in the game for just as long and most likley for A LOT LONGER
Your link is more of this narrative that became "popular" right about the same time that use of the UK sizing system became popular, popular outside the UK anyway.
That narrative being that "most women are wearing the "wrong size bra" "
Also confidently enough right about the same time, the early ))'s that this narrative started that women in the EU, in general, and the UK in particular were so much more "busty" than women in the rest of the world and in particular women in the US, where teh average for decades, was a C cup, 36C to be precise. No we were suposed to belive that virtually every woman in the UK was at least a D, or DD and everybody and her sister was an LDM clone, H cups, why they were not "rare" AT ALL even though "the eye test" said otherwise.
That women here in the US for their entire lives have been wearing bras that have 4 inches of "slack" as far as teh band size goes .
Do you have any idea just how "loose" of a fit that would be ?
Think of it like this
You wear size 34 inch waist jeans, not super tight, or even "snug" just fit to the point where you do not need to wear a belt to hold them up and then you put on a pair of 38 inch waist jeans, you know GD well that they would be loose to the point of "falling down" if you were not wearing a belt to hold them up.
Now you enter a woman who you say should be wearing a 30 inch band size bra and she puts on a 34 now before you even consider that that 4 inches represents a greater % of 30 than the 34 in my jean example do you realize just how loose of a fit a 34 inch band bra would be on her ?
Two fingers ?
How about an entire hand ?
Now I and I am sure you could find plenty of guys in the US who would say much the same thing, have been taking bras of women for 40 years and taking them bra shopping for well over 30. women from 4'10" to 5'10, from 90 pounds to 190 pounds, from A cups to H cups, natural and enhanced, in a few cases REALLY ENHANCED and almost all of them where wearing bras that would have "passed" that "2 finger test", in anything they were on the tight side, bras (because I looked or bought them) that were 34 t0 38 band size, a FEW 32's, and NO 30's, certainly NO 28's NONE were "loose" and had an extra 4 inches or for that matter even 2 inches of "slack" like you would would experience in my jeans example.
Christy Canyon 5'7', 125 36DD
You would have us believe that she has been wearing the wrong size bra for like 50 years now ??
In all that time you think she NEVER got "properly fitted" ??