opaque apart from that you should verify that the JWT token is issued by a trusted party and fulfills claims that your service requires? :P (Unless those parts are handled by a library you've configured to check this).
The point is that when logging in with AzureAD your app needs to talk to AzureAD using OIDC but JWT is an implementation detail of that specifically and there's no reason this means you need to or should be using JWT throughout the rest of your architecture.
If you're using AzureAD for IAM, you're not "using JWT for authentication tokens", you're using AzureAD. This isn't what the article is about. The article is about building your own services that generate and process JWTs. And yeah, if you use an OIDC API you likely use an OIDC library instead of rolling your own. So as far as your own code is concerned the token is entirely opaque.
