I guessed that from context and was just making fun of it. Writing it that way is definitely confusing and shouldn’t be done. "mm" is internationally recognized as "millimeters". In financial papers, sometimes, Roman numerals "MM" are used to indicate millions, but then it should strictly be capitalized. In my opinion, for general use, number followed by a single "m" conveys the idea best ($40m), and followed by "k" for thousand ($40k). Use capitals for more formal settings ($40M and $40K).
For sure! I hate the ambiguity that it adds. MM in upper or lowercase is something I've seen for millions in financial and business contexts, along with a single m for thousand. To avoid ambiguity I ended up using K for thousands and MM for millions back in the day.