Mel float is a double precision c float. See mel isnt a programming language per see* so it only exposes one type of floating point numbers, name has nothing to do with c naming conventions. Also what the default prints show has nothing to do with precision. Maya just allways prints values in short human readable form doesnet mean the value is stored in that form.
Proof (now insetad of assuming you could have tested in 2 seconds):
float $a=0.1234567890123456;
float $b=$a*1000000000;
print $b;
123456789.012346
IF it would be a float it would have approximately 7 digits available now it DOES clearly have atleast more than 15
Either way the question is totally irrelevant you use DAG to store values. This is the sort fo things one expects people who think about mel in classical programming terms would ask tough. Simply Mel is not used that way and the sooner you realize it the better for you.
*mel is a journal language its purpose is to make GUI functionality (since its journalling mostly) and DG connections traversals and dis connections. The real language of maya is the DG to which mel acts as a front**, and the API as a programming framework.
** this leads to countless weird things where users ask about how to do something in mel and the answer is a flabbergasting you dont, but at the same time the demo does it it in mel since the operation is ACTUALLY done by the DG. So something may at the same time be impossible to do in mel but dooable in mel by leveraging on the real data manipulator the DG. Theres lots of unfortunate examples regarding this, and unfortunately even seasoned maya users may NOT GET WHAT MEL IS ABOUT. So even best of learning material makes the worng assumption at step 1.