Is it? I wouldn't say ironic, however I can see why you might reach that conclusion.
The demo isn't a representation of a productionised end product. It is a playground representating the findings of the authors article for desktop users; it simulates a dynamic window which you can resize using your mouse - so the demo not functioning properly on mobile seems acceptable.
How's that? It's an article about fluidly resizing typography based on viewport size: it would make sense that a [proof of concept] demo tool would be built to work in an environment where you can actually change viewport size. How could you possibly make use of the tool in an environment where you do not have the ability to do that?
Then what's the point of the demo? Dragging the edge of the browser window make type scale fluidly? Typically the user is at a fixed viewport size, or they change to one or two other ones when the window is resized.
But from a practical point of view, if you are building a design tool (which is what the demo is), and the purpose of the tool is to apply adjustments across screen sizes, it needs to be in an environment which allows for different screen sizes. The entire point is that you can adjust how the type scales based on viewport size: if you cannot scale the viewpoint size, the tool is effectively useless. The end target of the code produced by the tool is not the same thing as the environment the tool runs in. The vast majority of GUI design tools are for desktop, for what should be very obvious practical reasons.
Like, Adobe XD (to take one prototyping tool) doesn't work on mobile. You can view the end prototype on a mobile device, but the tool itself does not work in that environment, and it would make no sense for it to, the environment is too restrictive.
The demo isn't a representation of a productionised end product. It is a playground representating the findings of the authors article for desktop users; it simulates a dynamic window which you can resize using your mouse - so the demo not functioning properly on mobile seems acceptable.