When learning kayaking you have to practice a water escape, I wish there was something similar for driver's ed in the Netherlands. Thinking about it is terrifying.
I think it's relative, right? That's how abstractions and interfaces work.
I can write a module with integration tests at the module level and unit tests on its functions.
I can now write an application that uses my module. From the perspective of my application, my module's integration tests look like unit tests.
My module might, for example, implicitly depend on the test suite of CPython, the C compiler, the QA at the chip fab. But I don't need to run those tests any more.
In your case you hope the in-memory database matches the production one enough that you can write fast isolated unit tests on your application logic. You can trust this works because something else unit-tested the in-memory database, and integration tested the db client against the various db backends.
I know someone who uses it to design clean drinking water distribution systems in rural communities in Central America. They would not be able to do what they do if they had to pay for an expensive commercial licence.
Desktop EPANET is still windows-only though, so having a browser version is pretty cool.