I think the point is that even if OP can sustain themselves, leaving before having a new job lined up will likely negatively impact their chances for their next

I think the point is that even if OP can sustain themselves, leaving before having a new job lined up will likely negatively impact their chances for their next job. So unless they can sustain themselves indefinitely (unlikely, else why would they have a job now), lining up a new job before leaving seems like good advice.