Miss Adebayo visited and said something about grief, something nice-sounding and facile: Grief was the celebration of love, those who could feel real grief were lucky to have loved. —Chimamanda Ngozi Adichie, Half of a Yellow Sun
“Joker” takes off from a facile premise and descends into incoherent political trolling as a result of scattershot plotting and antics—its director, Todd Phillips, appears not to see what he’s doing. —The New York Times
Unless you read a lot into definition 4 of Webster's, the app dictionary, or even the word 'shallow,' gives a result much more accurate to how I've seen the word actually used. With more than a century and a half since the dictionary was first published, seems like plenty of time for a shift in meaning to happen.
This meaning has diverged from French/Spanish, where the word still just means "easy".
It is pretty common for words to diverge in meaning and drop old connotations as they find distinct niches.
In Latin (and thence English) facultas meant ability, while facilitas meant easiness.
The adjective facilis comes from the verb facio (to make or do; from the same PIE root as the English verb 'do'), and meant something that can be done/made; something easy, ready, or quick; or someone friendly, courteous, or compliant.