Although in modern times it's almost ridiculously hard to avoid added iron in food these days due to enforced fortification based on outdated ideas from the 1940's.
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.