250.501(b) doesn't say what you say it does. It says if you are in breach, the landlord can send you a 15-day notice to get out. It doesn't say that this is how much time either side has to give as month-to-month tenancy termination.
Since your lease says 30 days, 30 days applies. If your lease didn't explicitly give a notice requirement, the case law says that the tenant isn't obligated to provide any.

