I know you said you are using the right acad.pgp file, but this will absolutely open the one AutoCAD is reading for the current session of AutoCAD
(startapp "notepad" (findfile "Acad.pgp"))
Once you get back there, is *YOUR* XL alias the last XL definition in the file (in the user section at the bottom)?
Lastly, any autolisp definition of XL will override a PGP definition of XL.
Make sure there are no autolisp definitions of XL present.
At the command line type in !C:XL
If you get anything other than nil, then there is a lisp function named XL out there somewhere.
Code:
Command: !C:xl
#<SUBR @0000000036334f20 C:XL>