They mix 225g sand with 50g of corn starch.

It's more "sand based" than "corn based".

in concrete, cement makes up less than 25% of the mix, and here corn is like 20%, so I think when you're comparing this class of materials (concrete, epoxy granite, concrete), it makes sense to describe in terms of the binder even if it is a smaller % of the complete product.

All concrete is more aggregate than binder… but it’s the binder that matters from an emissions perspective.

Well, tbf the paper clearly says it was published in 2017 and the first author is the same person who uploaded the presumably experimental stage video to YouTube in 2015.