Yeah, the thing that kinda annoyed me about the article is that it even acknowledges this fact ("even though life expectancy at birth as I’ve used here isn’t the best proxy for this"), but then for some reason refuses to make the next rational leap that there were plenty of great grandmothers in previous generations, totally invalidating the article's main thesis.

Obviously there have been huge changes in family size, parental age at first birth, etc. over the last few decades. I'd argue the lack of great-grandmothers is going to be the least consequential of these changes.