Yes, really. Your own link confirms it at the top:
> The standard financial abbreviation for million is MM, e.g. $300 MM.
If you use MM, basically everyone will understand. If you use M, some people will assume million but anyone with experience in finance (or anywhere close) will read "thousand".
I pretty sure the blurb at the bottom of your link is wrong, though. 'M' as an abbreviation doesn't come from Roman numerals. That claim is all over the Internet, but I don't believe it's true. The 'M' is for mille, meaning "thousand". We use this in all sorts of terms, like millennium, millipede, "per mil", etc.
