> why would they format Months, by "L" instead of by "M"

"M" is used for months in Luxon, and is usually what you should use. "L" is only for standalone months, i.e. not within a date string, usually not what you want. In English these are the same thing, but in some languages they are different.

https://github.com/moment/luxon/blob/master/docs/formatting....