It's possible.
The caveat, as you say yourself, is that not everyone.
So how many? As you'd probably guess, a small minority.
I think the coaches that believe anyone can do it are very bias. They may have been talented, motivated, constantly coached youths, and now as adults they have only coached motivated, talented, youth. And the only players they see day after day are serious players at national tournaments.
Let them take 100 random students. Let them take my mom, your cousin, the guy on the bus... let them take the person who calls it "chest" and the adult who has played for 10 years without knowing en passant.
After this I'll let them report to me how possible it is.
But!
Ok. So you're not the type of person to wake up in the morning and squeeze 5 minutes of analysis in before breakfast because it's a joy to do. You don't have much free time either, you're a regular guy with a job and family. So you get on a program and then what? Probably not expert or master, but you'll no doubt improve quite a lot. And probably enjoy it too.


I just don't know, as the article states, that it is probable to hit Expert or Master in 5 years. To be fair, the writer does not say that it absolutely will happen, but that it is probable, after 720 hours of instruction, 2600 hours of study, plus 10 hours of homework per week.
Do you think it is probable to hit Expert or Master in five years, taking into account the hours of study and lessons?
Keep in mind, only 5% of USCF members attain Expert (2000-2199), and only 1% hit Master (2200+).