Mxy twins = regular, mainstream Mxy, so not sure why the unnecessary distinction.
...unless you want us to consider feats exclusively limited to the Mxy twins? in which case, it's causing some minor retcon and some planetary level feat. Not much to go by.
Well, how often does a comic featuring a mainstream character state that this is the mainstream character rather than an alternate version? ...
Anyway, there's nothing to suggest it was an alternate version. Also, Superman said to the twins that he liked him as the funny little imp (indicating that they are mainstream Mxy):
Not to mention that there have been references to the twins. In Adventures of Superman #630 (12 issues after the Mxy twins story), Mxy indirectly references the twins: