Ok, i will say this again. There is no way you can predict a players skillset. You can study match ratings and stats from 2 seasons if you like, but you will never get an accurate reading of a players strengths and weaknesses in his abilities. Therefore the best you can do is work out a players rough ability by his salary, assuming that he is evenly balanced (possibly factoring strengths if stats consistantly favour a certain tendancy ie fg%, fg3%, assists etc).
Since the best (although flawed, still the best) method is a lot of relying on the opponent being relatively balanced, the next biggest factor into a players strength is gameshape. Now it is known to you that gameshape and DMI and salary are relative to eachother, and I know that the game manual plays down DMI (it also says 2-3 zone is increased inside defence even though there is a large number of people who say it is useless), but I will try my best to explain how I use them all together as a scouting tool...
1) DMI=salary calculated by the code. Correct?
2) DMI is affected by gameshape, which is a major factor in a players strength in a match. Correct?
3) DMI is affected by a players skills increasing or decreasing. Correct?
If DMI was only modified by training, then why is it affected by gameshape? When you do the math, it does not always come out exact, but when you consider that gameshape has sublevels it is very close (especially since it is more consistant at Proficient which is the highest point). Now a players DMI is also affected by a pop in skill according to what it would make the players salary if it was reset that day... So with 3 different factors related to a players strength, how do you not see DMI as a guide to how strong an opponent is on gameday?
Lets take your original example. Player B drops to 440000 DMI opposed to 380000 DMI in your opponent. If those players were evenly balanced, I would therefore predict that the difference in gameshape would have made the contest between these 2 players almost a dead heat. Yes, player B would have his arse handed to him, but in the world we live in nobody is priveleged to such information when lining up against an opponent. Now in my experience I would say that the difference in gameshape and the percentages that my theory calculates strength would be pretty much on the money. Feel free to do the math yourself on a few players and see if im making it all up.
So in conclusion to your disbelief of the quote...
So in saying that, not only would I say DMI is an indicator of how good a player is, but I would say it is the single best indicator available!
I would say that there is enough evidence there to prove DMI is an indicator of how good a player is (as also stated in the quote from the game manual), but also just how effective it is in getting the average strength of your player against the strength of your opponent on matchday... If you believe you have a better available system I would like to hear it.