Lets jump back a little bit, this is what I saw.
So originally we had developers and they did all the testing themselves, they had full responsibility for quality, it was fast and efficient.
Products got more complex, larger, more integrations with other systems and being human, the developers let a few things through the gaps. The industry often led by big financials overreacted a little bit and brought in separate test teams to test at the end.
So here in effect it was waterfall. We had things like PMI running how projects were run but that training did not really talk about testing at all, but a lot of new test training came out based on the PMI command and control model. In my view bad testing practices came out of this, over emphasis on planning and test cases, QA police mentality, limited collaboration and a bias towards tickboxing written requirement specifications.
Byproduct was also developers started taking less responsibility for quality, blame game and a lot of other nonsense alongside potentially poor testing itself.
Testing generally testing in any context but the above often gives waterfall testing a bad slightly unfair wrap. You can do good testing in waterfall just as well as agile.
Those testing problems with waterfall were just a small part of an adjustment of the testing model to adapt to agile development practices.
The original fast and efficient view of developers doing test activities was re-embraced, alongside high levels of collaboration and team ownership of quality, but rather than revert fully back to original model it also embraced complimenting the team with small numbers of professional testers. This model can also be applied to waterfall practices.
The comparison of agile versus waterfall testing is flawed but there is some thinking bias behind it.
With regards to developers doing test activities that was/is a fundamental part of their role from a product efficiency perspective. From my perspective I expect them to cover the known risks and the basic verification side of things allowing my testing to look more into the unknown.