Apologies in advance for relying on the @ArseniMourzenko comment, but once I started to read the Wikipedia I immediately understood what Canonical Schema means.
Here OP's comment that focuses on the real doubt
I believe even in microservice architecture, the request and response
have to comply to some data model.
Some data model yes, but seems that the article is referring to a "shared" or "common" data models between 2 or more services.
The Canonical Schema is a pattern meant to save services from in runtime data transformations. It also saves you from duplicating code. But you are then coupling your service to an external data model too.(See diagrams at the Wikipedia's page linked above)
It's a sort of common "language" between services.
So looks like the article is emphasising on the total independence of the MS from the "ecosystem" where it lives at.
Take for example the mention it does to ESB.
They also very much avoid using ESBs and instead implement ESB-like
functionality in the microservices themselves.
ESB usually demands an enterprise data model (messages) which is going to be common for everyone attached to the bus.
So, back to the article, seems that the author is pointing to the fact that MS rejects to be attached to any external system (and their constraints).