The difference between devs and dedicated QA people is that devs know the dark corners of the implementation. They know the edge cases and scenarios that they struggled with getting right. That's where most testing focus has to be. QA doesn't know any of that. They can play through some scenarios that are the expected ones from the spec, perhaps have some hunch of what could be the tricky cases, but they don't actually know.
I love seeing all those downvotes for my GP comment. That's all the people working at companies where stuff that devs feel is below them is dumped on QA people, which is the main reason my company is well-known for shipping top quality (without having a QA dept) while our competitors struggle with quality (despite having a QA dept). That's all the evidence I need to back up my claim.