Yes it was.
> Think about it, how could you possibly code an entire app and only then debug it?
Which is why programmers hate waterfall so much, but you're absolutely wrong to think this isn't exactly what was being attempted. Time and time again, management in an attempt to cut the cost of programming time, thought the way to do it was to first build specification for everything and try and prototype the entire app up front, storyboard every screen, build out mountains of specs because naturally they think that's how you build things, with detailed blueprints so all the decisions have already been made. They couldn't be more wrong, but it's certainly the most natural way to think if you aren't a programmer and don't know better.
> 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.
Agile in general had nothing to do with testing, the big change in agile was removing the big design up front, the specs and meetings and months spent planning something before being developed. Some agile methods like extreme programming certainly had testing as a big part of their process, but what differentiated agile from waterfall was introducing iterative programming where work was done in short week or two cycles and then delivered whereas waterfall wastes enormous time trying to nail down details that simply ended up being wrong come programming time.
tldr; agile is not about testing, it's about iterative development in short cycles with little planning and always has been and that's what made it different from waterfall; no "Big Design Up Front".