All our testers are coders (i.e. devs), because the tests we use are pretty much all "automated tests" driven by software which has to be coded.

Manual testing is useful for testing a user interface to see how it "looks and feels" (also sometimes called UX testing for "user experience"), but comprehensive testing is best done by software. Doing it in software means you have tests that you can re-use over and over again too, to detect if something gets broken. Human testers won't remember to test everything, and they ultimately become too expensive if you want to do tests every time the software changes.