On my first serious project at the first job I estimated back-end part of an app as 75% and front-end as 25%. It ended up exact reverse of that. And in the next years the pattern repeated over and over.
Everybody seems to think that UI is just about placing buttons but backend is the heavy lifting. In reality, requirements change more often and in a more fundamental way, code is harder to test, more third party dependencies involved, even the dumb KLOC metric ends up bigger.
By any mean, low-level backend will be harder than high-level backend, and low-level frontend will be harder than high-level frontend. Try implementing HTTP server in plain C vs. plain Java (no third party frameworks) and you will see. Being easier is practically the definition of high-level, he is just comparing things that are too different.