As a developer with an arts background (studied music, happened into tech work after college through years of mostly self-taught hobbyist tendencies) I'd agree that the design of these tests themselves is a factor that's
thoroughly understated.
Timed algo challenges encompass a slew of antipatterns in terms of how good code is actually written and shipped. To begin with, pitting someone against a clock and hidden test cases (and a foreign editor) is actively optimizing against solutions that are readable to other human beings -- or to the person writing them, a year from now. The nature of running them in a browser means that it can't evaluate a person's capacity to actually use tools outside of core language functionality. Never mind that building the entire exercise around predetermined test cases precludes any way to gauge whether the person taking it has an understanding of writing tests.
And that's assuming your test environment doesn't add obnoxious and arbitrary restrictions of its own. Like telling you that using documentation is cheating. (Btw imagine listening for ctrl+t here, but not ctrl+n.) Or offering you "the language of your choice," but then throwing API call exercises at you while limiting your choice of JavaScript runtimes to a bare installation of Node -- the only one still in active development, out of a list that also includes every browser you would use to actually access the test -- that doesn't support Fetch.