The memorandum does not need to specify every detail pertaining to those visa classes.
The primary M-class is established under the INA at 101(a)(15)(M)(i) with the following language:
INA 101(a)(15)(M)(i) an alien [...] who seeks to enter the United States temporarily and solely for the purpose of pursuing a full course of study at an established vocational or other recognized nonacademic institution (other than in a language training program) in the United States [...]
The definition provided for "full course of study," at 8 CFR 214.2(m)(9) as aforementioned, states "A 'full course of study' as required by section 101(a)(15)(M)(i) of the Act means [...]"
It's a fine point but a night qualification falls short of the threshold meaning it cannot in the overwhelming majority of cases, on its own, be pursued under an M visa. Unless one argues that the minimum time requirement can be pro-rated. The rule should be modified.