I hadn't personally heard much about Envoy's universal API before this, so I had to look it up a bit. Found a great blog post by Matt Klein (king of Envoy!):
https://blog.envoyproxy.io/the-universal-data-plane-api-d15cec7a
While the goal of providing an experience (API) for configuring and controlling things across many environments and providers is the same, the space that they are each focusing on is very different. Crossplane is focusing on delivering a declarative API/experience (same as Kubernetes) for scheduling, deploying, and managing workloads across cloud providers and on-premises. Applications (container, serverless, etc.) and their dependencies (databases, buckets, message queues, etc.) are the focus: all the useful components that make up a complex application.
Envoy's realm is lower level networking, service mesh, service discovery, etc., so I don't see much overlap between the two. Maybe complementary in fact. I think there's a lot of networking config/setup that needs to be fleshed out for Crossplane in order to connect stateful workloads running in different geographic regions and providers, maybe Envoy would be able to help out with that?