The source was the Oxford English Language Dictionary. Merriam-Webster concurs.
https://www.merriam-webster.com/dictionary/hearken%20back
Well, when I clicked on your post, it said “Oxford Languages,” which I never heard of. Anyone can use the word “Oxford.”
I realize things may have gotten worse, since I left in-house book publishing 21 years ago. I knew things would get worse and not better.
You’re probably too young to remember when Winston cigarettes caused a major debate by saying “Winston Tastes Good LIKE a Cigarette Should.” No more major debates. The travesties just become part of the language.