Subparsers have aliases, argument option strings don't, at least not formally.  Once an argument is added, its flags are entered in several lists.  One list belongs to the Action itself, another belongs to the parser (it might actually be a dictionary).  The code could check that '--a-b' and '--ab' belong to the same Action, but I suspect it doesn't, at least not at the point where it is checking for ambiguity.

We can look at the code to verify this.  It is also entirely likely that this issue has already been raised.  There are other issues regarding how it checks for abbreviations (and how to disable abbreviations).