An alien intending to embark on any non-trivial "course of study" must do so under an M- or F- visa.
What they are saying is that Flight Training leading to a qualification requires the visa - period. The letter that I pointed to above makes no reference to the distinction between "short" or not - simply that a vocational course that leads to a qualification requires the M1 visa.
An M- or F- visa requires the alien to engage in a "full course of study."
For an F1 visa - yes because those folks are attending one of the handful of academic schools for 2 years or so - but not for an M1.
Plenty of folk come to the US to attend one of the Florida based schools and get their Private in 21-28 days. Even allowing 55 hours to complete - it only averages 14 hours or so a week - but they still require a M1.
It is still a "full course of study".