It’s even worse than this. From Wikipedia:

The chickenpox vaccine first became commercially available in 1984.[10] It was first licensed for use in the US by Merck, under the brand name Varivax, in 1995.

So in a way it being acceptable 30 years ago does mean something. Not what the author thought though.

My understanding is that it’s for learning only. You should be safe for your private singing purposes. It seems like you would owe them royalties if you created your own song lyrics, after having read/heard one of theirs…