Pure waterfall as it's depicted by Agile consultants was never a thing. Think about it, how could you possibly code an entire app and only then debug it?

However, there was a waterfall-like process that was widely used. After getting the reqs and design in place, you started coding and you mostly coded. You would compile as you went along and made sure that your code worked on the handful of tests you put together as you went along. When the app was mostly coded, you then started more serious testing: writing more elaborate tests and verifying functionality. This step often highlighted buried errors that you then spent time debugging, writing additional tests, and so on.

So, the biggest innovation in Agile was the latter's orientation towards developer testing concurrently with coding. Which, of course, has had many other ramifications.

While the old way of doing things might sound klunky by today's standards, when done right it actually worked better than its reputation. Because devs hated spending hours in the debugger, they tended to code very carefully. The concept of "let's code this and run it through some tests to validate it" was unknown. Rather you coded so that you were pretty much sure that what you were writing worked properly if it compiled without error.