The New Yorker prefers insure to ensure. They have a unique house style. I commented on another thread about alternative spellings like vender instead of vendor, too.
That M-W entry literally says they're different words with different meanings:
> They are in fact different words, but with sufficient overlap in meaning and form as to create uncertainty as to which should be used when.
> We define ensure as “to make sure, certain, or safe” and one sense of insure, “to make certain especially by taking necessary measures and precautions,” is quite similar. But insure has the additional meaning “to provide or obtain insurance on or for,” which is not shared by ensure.