The question is, when it is wise to do so. Changing the public interface of a "shared module" or published "library" comes with a liability. Code reuse is almost a no brainer in the case of a standard library that rarely or never changes. Using a permanently changing modul whose modifications provide no real benefits for your project makes no economic sense. The sweetspot is somewhere in between.

What the parent actually did when copying the library/module code was creating a sort of a a long-term support version for himself maintained by himself. His cost-benefit analysis told him that this is better than always trying to keep up with the changes of the library/modul.

My comment was aimed at setting the bar rather high for when to share reusable libraries. And even for my own modules I rather prefer copying code instead of reusing it in different contexts. There is a reason why the contexts differ and it is typically easier and more economic to adress this differences in its particular place instead of preparing for them in shared code.

The more up-stream a library/module is, the more general it must be, but the less liable to change it should be.

With monorepo he wouldn't have had to worry about this decision, since the decision to go back and forth would have been much lower cost. It's low cost decision either way and low cost impact if you make a wrong decision.