I’ve never seen ‘mm’ nor ‘mn’ used in British English, where ‘m’ is a common abbreviation for “million”. But this might be a localisation.

Wikipedia does recognise ‘m’ as an abbreviation: https://en.m.wikipedia.org/wiki/1,000,000

It’s definitely open to confusion but generally if you already known that abbreviation exists then one can usually deduce how to interpret that abbreviation from the context of the sentence. In this case, it’s a financial headline so I assumed it was “million”.

> Wikipedia does recognise ‘m’ as an abbreviation

The lower-case m is used in British English. Upper case M can be used, but it’s unnecessarily ambiguous and close to wrong in American English.