Yes. If you look at the ZILF compiler in particular, which is capable of compiling the original sources, there's a lot more MDL in there than you'd expect would be required for ZIL proper.
Right - that's because ZIL was more or less a _superset_ of MDL.
ZILCH (Infocom's compiler) provided all the functions of MDL, _plus_ a bunch of new ones that manipulated data structures which were then used to generate assembly code for the Z-machine.
One of those new functions, ROUTINE, accepted code written in a domain-specific language resembling a stripped-down MDL, which was then translated into Z-machine instructions. But that domain-specific language isn't synonymous with ZIL: other functions that were inarguably part of ZIL, like OBJECT and SYNTAX, are not part of that domain-specific language.
IMO, the only reasonable definition of ZIL is "the language accepted by a ZIL compiler", which (depending on whether you look at ZILCH or ZILF) is either a superset of MDL or an overlapping set.
