PhD supervision in the Netherlands: what ranks can supervise?
There is a difference between supervisor and promotor. You need a promotor, who is going to sign off your thesis. This needs to be a full professor (not associate or assistant). However, your main supervisor can be any PI (comparable to assistant/associate professor). Your daily supervision can even be handled by postdocs, and to some extend even by senior PhD students.

Depending on the status of your PI, if he or she isn't full professor (yet), a promotor may not be involved in a PhD students work at all.