That page says MM is antiquated, falling out of favor, and M is the more modern way to represent a million.
You have to be careful about separating your interpretation of what something is saying when referring to the claims of the content directly like that. Nowhere does the page say antiquated, that's just one particularly strong interpretation of "becoming less common". It'll probably eventually be antiquated but it certainly isn't yet - it's still very popular and accepted.
Because finance has weird conventions about numbers, and $7M could be misread as $7000 in the wrong context