But in this case the company is someone who claims to be "The best way to create and share a grocery shopping list and organize your recipes."
Sharing is part of the deal with them and a sign in process that from the start complicates it is understandably a no go / introduces all sorts of complications that they detail in the article.
If the goal for the app is for itself to be a tool for identity management, then knowing my email is especially not needed...after all, all identity context is already in the app!
UX should center around ease of sharing some hash value across some other medium, not "searching by identity."
I do honestly empathize with the app creators. But anybody choosing an obscuring email by definition does not want to be identified by email.