Unless your challenge literally stops at, "make X appear onscreen" with no regard for quality, testing, etc. Giving unchecked/unverified time restraints isn't fair. It doesn't matter you're giving more time than it should take to complete. If the task can be done in 2 hours, but you give "6" and Candidate A does it in 3, but Candidate B does it in 32 (but tells you 6) you're ranking two totally different submissions. Candidate B might have a super polished submission, while Candidate A has a baseline submission.
The poster you replied to was suggesting that tests should be either 1) not based on quality of submission and simply rely on difficulty so that only a few candidates can complete them or 2) based on quality and difficulty, but with a checked and verified time to keep the playing field level. However those options are both at odds with "low stress."