From what I found, healthy can have the meaning of healthful, moreover the Oxford dictionary consider them as synonyms since 1552 (according to an unverified source). However, there still seems to be a big fuss about healthy being used in place of healthful.
I did not know about healthful until fairly recently and have been using healthy with the very meaning of healthful in a lot of occasions without being told that I was off.
Therefore, I would be glad if you could show evidence that healthful is not a word redundant with healthy.
Nota: I am aware that there are two questions addressing this very problem on ELU but the answers were lacking solid conclusions.