I don't know why, but I thought there was a good reason for the presence and use of a native replacement. Maybe when the readdir command was developed, it was in the Tcl 8.2.x area and glob didn't take the -directory option.
As I recall, that (and other missing functionality) was why I originally added it. Since that's no longer an issue, there doesn't seem a need for readdir() in pextlib.