The article doesn't have a single reference to a research paper. It's entirely possible to write a similar essay with an opposite title like "The research is clear, let's keep elementary homework", and it would be just as convincing.
I think you mean it doesn't have a single reference to the paper.
This is why it's detrimental to have all these Journal articles locked away from public view. A researcher can make a claim that influences public opinion, but the general public must pay a lot of money to actually review what the claim is based upon!
The paper is, incidentally:
Cooper, H., Robinson, J. C., & Patall, E. A. (2006). Does homework improve academic achievement? A synthesis of research, 1987-2003. Review of Educational Research, 76, 1-62.
Luckily there is a copy of that article you can find here:
No strong evidence was found for an association between the homework-achievement
link and the outcome measure (grades as opposed to standardized tests) or the
subject matter (reading as opposed to math). On the basis of these results and
others, the authors suggest future research.
Cooper compiled 120 studies in 1989 and another 60 studies in 2006. This comprehensive analysis of multiple research studies found no evidence of academic benefit at the elementary level. It did, however, find a negative impact on children’s attitudes toward school.
The 1989 reference is the book _Homework_ by Cooper, and the 2006 reference is "Does Homework Improve Academic
Achievement? A Synthesis of Research, 1987–2003", Cooper et al 2006 http://upload-community.kipa.co.il/819201525856.pdf
If you are going to only have two research paper/book references, comprehensive meta-analyses are good choices. FWIW, the claims are consistent with what I've always read about homework research and there's no contradiction in https://en.wikipedia.org/wiki/Homework either.
edit: a typo