Right. As the top post also points out. There's no way to distinguish incoming UDP traffic as "spoofed".

My question is does anyone filter UDP egress based on source IP? Is there guidance somewhere that tells admins to do this?

Let me put it another way: If it was a workable solution to get admins to do this - to filter outgoing UDP based on source IP, then why are people trying to get network admins to change their DNS server settings as a way to reduce the possibility of DNS-based DDOS? That seems like a far more difficult task given that there hundreds of thousands of open resolvers and most admins understand working with firewall rulesets better than DNS configuration.