This is not a "random alternate convention", it's a highly used convention that you aren't familiar with. I wasn't familiar with it, either, until I moved to the Silicon Valley. Now I see it all the time.
There exists a world of finance outside of Silicon Valley. M and MM to indicate thousands and millions are a finance convention, not an SV one, people in SV just happen to talk about finance a lot. Anyone in finance would be familiar with it, and given that the amount here is a dollar amount, the use seems perfectly fair.
I strongly suspect that the page's etymology is wrong -- or more precisely -- not entirely correct. The Latin word for "thousand" is mille, which why the numeral for a thousand is M.
No one would be so asinine to mix numeral systems. Same thing with percent, which originally was rendered as "per-cent", as in "per hundred".
I completely agree with you, while I know $40+mm means $40M it is only because of context, not because I actually parse it as "thousand thousand". In my mind mm is millimeter, even as an American.
HN is the only place I see this notation used.
The great thing about standards is there are so many to choose from.
Can't we just use SI prefixes or spell the numbers out instead of using some random alternate convention?