So only Eufy can answer this, I’m just a geek who knows what is possible.
Yes I think that can be done as a software upgrade. They’d have to cache some metadata which meant, say, you factory refreshed your phone, you’d have to go back to the server, as most domestic internet IP don’t change frequently, it could fallback if server unresponsive.
They not shared the technical architecture, specifically I can’t tell if they used uPnP to open a firewall port or not. Only a Eufy owner can tell (look at router, open ports). If they have then it’s written to only work when server up.
To make it totally serverless, I think is a rewrite and take longer, but they certainly should be able to tweak it to handle a server hiccup.
In my opinion FWIW.