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".
Using Roman numerals for anything is a waste of everyone's time.