I agree with the general idea of sticking to your principles.
On the other hand, when it comes to your side projects, it truly is once in a lifetime that something becomes a global hit. A company or even a non-profit wouldn't hesitate to monetize this, and yet individual is the one who is given more scrutiny when they "sell out."
This simple little app being popular is something that could make a single developer financially independent (or just bring in a steady side income that gives you more career options). The author isn't exactly independently wealthy, he's still doing salaried work for a company. He is still (trapped, depending on your perspective) in the 9 to 5 like the rest of us.
It's not like Wordle is healthcare, something that is monetized that shouldn't be. It's just a game. A lot of people love it so much that they would gladly pay for some premium version of it.
Another way to keep the original product 100% free would be to open a store with merchandise or use something like Patreon.
I think of the Open Source movement not being about "freedom as in beer." It's 100% okay to sell open source software, but the point of open source is "freedom as in speech."
Monetizing is not the same as selling out or being predatory or immoral, because how you monetize is a very important detail.
Zach could stick to some kind of principles like those while still monetizing. Consider this as well: there's a good argument that not monetizing something as popular as Wordle is doing his family a disservice. All of our families, especially in a capitalist system to which we are subscribed with no opt-out, are basically little kingdoms that have to do their best to secure the stability of their future. Giving up a low-effort income source like this is kind of like turning away 10,000 soldiers who want to join your kingdom's army – it can only hurt you in the long run.