M-W's lexicographers openly discuss their process of descriptively documenting language as it's observed in real use. If usage of the word "cromulent" continues to grow to the point that it reaches the threshold for inclusion, it, by definition, belongs in the (descriptive) dictionary.
If "cromulent" has not yet reached their threshold for inclusion, then they themselves should not be using it in casual language. Contrapositively, if they are using it casual language, then presumably it has reached their threshold for inclusion as a word.
For them to casually use the word as though it were real after arguing that it is not harms their credibility as arbiters of what words are or are not real, despite the comedic value of them doing so here.