Why not "geen professor"? Is it like, I might be a professor (by occupation) but not the professor you are looking for?
- Ik ben geen professor - I am not a professor (Just not in general)
- Ik ben niet de professor - I am not the professor (Perhaps I am a professor, but I am not the professor you are referring to)
Is this American Professor = Lecturer or English Professor = Qualification after Doctor?
According to wikipedia (https://nl.wikipedia.org/wiki/Hoogleraar): in Belgium a lecturer, so anyone who teaches at a university. In the Netherlands, it's defined more narrowly: someone appointed hoogleraar by a university, a department chair, responsible for research and education of that department. This usually is a doctor, but this is not necessary.