Couldn't add schema extension plugin extend type of another service

I would like to add a schema extension plugin in this nodejs Postgraphile server to pointing to another type in another GraghQL service. The other service is running with Apollo Server. Then would like to register both GrqphQL with Apollo Gateway.
However, in my plugin:

```js
const { makeExtendSchemaPlugin, gql } = require("graphile-utils");

const SchemaExtensionPlugin = makeExtendSchemaPlugin(build => {
// Get any helpers we need from `build`
const { pgSql: sql, inflection } = build;
return {
typeDefs: gql`
extend type Query {
ThingLinked: [ThingLinkedProduct]
}

type ThingLinkedProduct @key(fields: "id") {
id: Int!
name: String
product: Product
}

extend type Product @key(fields: "upc") {
upc: String! @external
}
`,
resolvers: {...}
};
});

export default SchemaExtensionPlugin;
```

Error:

```
Error: Could not find type named 'Product'.
at getType (C:\Workspace\EMCP\node_modules\graphile-utils\src\makeExtendSchemaPlugin.ts:516:15)
at C:\Workspace\EMCP\node_modules\graphile-utils\src\makeExtendSchemaPlugin.ts:755:24
```

I even tried to add Apollo's buildFederatedSchema API in this extension, got different error.

Is this feature supported?

I'm able to add 'extend type' the other way around. Added 'extend type' from here to the other Apollo Server GraphQL, and Apollo gateway can query this one using nodeId