It isn't about test coverage, it's about feature completeness.

For eg. One feature require backend & frontend work, after a week of work engineer says that this is done. So you ask him to demo, and it turns out he is doing a backend demo with some dev tools.

From a product perspective nothing has changed. Top engineers understand, that no one cares that only one part is working, they know the end user and will not bother managers/product owners with partial solutions.