The MitD outwitting Xykon.

[Goosefeather]

In this case, yes, it is:

A deus ex machine... ...is a plot device whereby a seemingly unsolvable problem is suddenly and abruptly resolved, with the contrived and unexpected intervention of some new event, character, ability, or object.

We don't have an established comprehensive set of abilities for the MitD. This is not the norm for a literary character. If Roy were to suddenly shoot fire from his eyebrows to defeat Xkyon, that would be DeM, because the new ability is outside of his previously established skillset. This does not apply to the MitD, because we don't know what he is, nor what he is capable of.