Unit tests are never a tool to prove stuff to managers/customers/stakeholders. They are a tool to prove to yourself and your teammates that your code works and have some reasonable amount of confidence that it will continue to work when your teammate (who might be fresh college grad hire with no experience) changes it.

Testing has nothing to do with engineer ability or experience. Regardless of that you can always make a mistake. And even if you are a rockstar Sr dev who never makes a mistake, in the real world any piece of code is gonna get worked on by people of all skill levels and experience so the idea that "a senior engineer wrote it so we can just trust it" doesn't work even if one accepts the assumption that you can just trust the code from an experienced dev. I am against 100% coverage requirements as well (except for perhaps dynamically typed languages) but I would still expect a reasonable amount of coverage and that coverage include good tests.