That said, burnout can make working harder worse than working a sane amount. But that doesn’t fall on the employer as much as the employee.
That is highly dependent on when the new employee is added to the project. If it's done early (as a result of proper planning), it's most likely not gonna hurt.
But with reasonable amount of people, if you start with the full team at the beginning, you can assign different responsibilities to different people. For example, one can do database, one can do web API, one can do web design, one can do integration with other systems, one can build and maintain continuous integration; you can also divide the domain knowledge among multiple people. Each person can focus on doing their part in such way that the others do not need to understand every single detail, and only use the agreed API.
However, if you add a new person late in the project... all simple things were probably already done, and all difficult things require a lot of knowledge about the existing code, that other team members already know, but if they spend their time explaining it to the new member, then their own work gets slower, so the overall speed reduces. (And yes, given enough time, the new member would learn everything and become just as productive as the old members. But the thing is, there is not enough time left at that moment.)
I was referring to my 40 hours ending on Friday at 5, and not returning to work until Monday morning. If management expects more from me then they need to hire more people or lower their retirement or delivery expectations.