Technically yes, because Zelle is just the frontend, but banks can settle between themselves via ACH if they don't have another arrangement or a RTP (real time payment) setup.
To answer the original question, because Zelle is ran by a consortium of banks, they all trust each other enough to release funds immediately and settle up overnight if need be.
Zelle isn't the ACH or RTP though, is what I'm saying. Zelle is the real-time notifications and accounting, but the reason every bank needs to have bespoke support is because it's each bank's job to figure out how to do the actual funds transfers in the background after Zelle payments go through.
To answer the original question, because Zelle is ran by a consortium of banks, they all trust each other enough to release funds immediately and settle up overnight if need be.