I suspect the recipe actually is "connect to a
LAN port on the node". As you say, allowing the node to bond to something it sees out the WAN port would be a pretty serious security hazard. Once you've configured the node as a node (AP, not router) then it should treat all its ports alike from a security standpoint --- but it has no business doing so when not yet configured.
Having said that ... I've not tested it.
Keep in mind also that if you bought your units in a multipack, they come preconfigured to associate with each other. It's possible that that behavior will work even if the node's initial connection is out its WAN port. That's less of a security hazard since the main unit is recognized by whatever ASUS is using for the purpose (probably just security by obscurity

but it's better than nothing).