You're right. And the problem with that is that the only opportunities for 100% coverage happen when you can start a project that way with the buy in of everyone on it. That is only likely to happen when everyone is familiar with TDD and comfortable with it. That creates a substantial hurdle for a group to cross when adopting it for the first time.
I prefer the approach we used here (at a non-startup). We simply decided that we were going to use automated tests as much as possible on a new project. Everyone was encouraged to use them. Our coverage wasn't 100%. However, when you are committed to creating an automated test case to reproduce any bug regardless of how the bug was originally found, your coverage expands. Usually you can write variants of the same test to cover related areas.
I can't speak for every member of the group, but my total testing time went down slightly and my test coverage improved enormously. Full TDD is not necessary in every instance. Just because you can't immediately make the jump to full TDD doesn't mean that automated unit test tools won't make your code better, and your testing easier.