It is still possible for a meeting to start without you (the host), even with Join Before Host disabled.

If you have given someone Scheduling Privilege, which allows them to schedule meetings on your behalf, when that person joins a meeting before you, the meeting will start and they will be made the host.

This is typically not a problem, because the security recommendation to disable Join Before Host is based on preventing unwanted/uninvited participants from hijacking a meeting, rather than someone you have given scheduling privilege to.

After you join, the role of Host can be reassigned to you.