I expect senior fullstack developers to have senior level of knowledge and expertise in backend and frontend. I don't expect senior level expertise from junior full-stack developers.

For that matter if you were a senior full-stack developer I would expect you to be able to build a production level frontend as well as a production level backend (though maybe not operation-wise, I.E. deployments and configuration, depending on your background).

That still doesn't preclude having expertise. For instance I wouldn't expect a full-stack developer to know how browser rendering works or the difference between layout and paint events (but I would expect him to know to never access an element's size during an animation) - that's someone with an expertise in front-end performance.

But anyway that's not what I gathered from the article, to me it purports that back-end developers shouldn't do front-end work.

It all just sounds muddled terminology to me. You say you expect them to have "expertise in backend and frontend", but then you say you wouldn't expect them to have expertise like "know[ing] how browser rendering works". So what is it? :)

There are levels of expertise, even as a senior developer.

I expect a full-stack senior developer to be able to lift a webapp from scratch (frankly I'll probably expect it even from an experienced junior full-stack developer in today's PaSS world). I expect every full-stack senior developer to be able to get the job done in both front-end features and back-end features, as long as they aren't very specific. I expect him to be able to do so in a maintainable and relatively performant way that has structure and consistency behind it.

I expect him to know best practices for both front-end and back-end. In front-end that is for instance knowing that you don't measure dom elements inside animations.

I don't expect him to know the internals that led to these best practices (in this case how the browser works and the nature of the layout phase) nor when is it ok to break them. If best practices were followed but something still doesn't work it's okay to call an expert (for instance an expert on browser performance).

Specializing as a fullstack-developer means that you are also an expert in some field (for instance browser performance, or web animations, or graphs).

Perhaps a better phrasing is that since I don't really expect pure front-end senior developers to know it either, only those who specialize in certain fields, so really a full-stack developer is already a senior front-end developer and a senior back-end developer.

There isn't something that makes a front-end developer more specialized in front-end then a full-stack developer, it's just that he didn't work enough on back-end.