I think there's some truth to saying all things are an "illusion" in some sense if that's how you define "illusion". But I wouldn't use the word illusion.
Thinking of things as solid matter is just our mind's way of approximating the the nature of matter based on our experience. As humanity progresses in advancement and knowledge, we're able to take a deeper more precise look at matter and we come to find that what we experience as "solid" is just a repulsive force. But that's not the end all be all way of looking at it. It's just a more approximate way. There's still even deeper theories in the work that may change or advance this perspective. Look at gravity for instance. It's one of the four fundamental forces. However, if you study general relativity, you come to learn to look at gravity as spacetime curvature rather than a force.
One might say that it being a "force" is an illusion or that it's wrong to think of it as that. Nonetheless, it's still useful to think of it as a force for certain practical applications such as in engineering (depending on the type of engineering). Because it may not be important to think of it as spacetime curvature if the application doesn't call for it. Likewise, matter is "solid" when it's important for us to think of it that way. And it's interacting forces when an application calls for a more precise understanding. I don't say this in the sense that we decide the nature of the universe. I say this in the sense that our understanding of the universe will only ever be an approximation.
In the end, "solid" matter is an approximation. Or understanding of it as a repulsive force, interactions and interconnectivity, is just a more precise approximation, but it's still an approximation. So with that said, I think approximation is a better word than illusion.