My guess here is that attaching the namespace definition to the camelContext element triggers special handling from Spring. Since we don't have a camelContext for XML routes in SwitchYard, the namespace definition is not registered with a namespace handler. For now, I would stick with a Java DSL route for this use case. If you don't mind filing a JIRA, we can investigate this a bit deeper and see if there's a workaround for the XML DSL case or if we can add support for registering namespace handlers.
Just submitted a PR to fix this issue - doing same as what is done by CamelNamespaceHandler. You'll be able to add xmlns attribute on routes/route element with this fix so it can be used in the xpath expression as well.