And SI uses 'M' as a prefix for one million. Oxford defines 'm' as million(s) in a money context, Merriam-Webster includes 'M' as an abbreviation of million.
It seems odd that anyone would think $25m should be read as $25000. It's much more likely that I would read $25mm as 25 millimeters instead.
The problem is that there are multiple traditional usages, with overlapping meanings.
Capital M has traditionally stood for both thousand and million. In other words $20M and $20m is ambiguous, depending on what tradition you come from. $20MM and $20MM and $20k are not ambiguous.
It seems odd that anyone would think $25m should be read as $25000. It's much more likely that I would read $25mm as 25 millimeters instead.