Update on this: everything is now working. It seems that it was a temporary error on the Zoom side.
We didn't change anything, and it just started working again 12 hours later...
It looks like this issue is happening again.
Our account doesn't see any webinars through the Marketo integration. Luckily, I had a support ticket open with Marketo (good luck getting a response from zoom right now) and they confirmed there was "downtime" on the zoom end after it started working again.
"Following up on the case, I do see that your Zoom integration is working fine. We also received a notification that there was a downtime at zoom."
Unfortunately today, the issue has come back up and I can only assume its another issue on the zoom end. Let's hope this is taken care of sooner than later. Last time we waited a day before we could use the integration again.