Interview with potential supervisor for graduate program. Should I keep contacting other potential professors?
You should continue contacting and interviewing possible professors until you have an official offer of admission that you have accepted, especially at other institutions.

It seems like this professor is most likely willing to take you as a student, but only if you are accepted to that program (which likely does not depend only on that professor's opinion), and only if nothing else comes up including an applicant they like better, unexpected personal circumstances, changing financial situation in the lab, etc.

As you say, acceptance to the program is conditional on having a supervisor: that means it is necessary for admission but not sufficient. The professor has only granted you permission to identify them as a potential supervisor on your application, this is not an offer or promise of admission.