It’s worse than that. Only naive reductionism expercs that you can always explain am X in terms of something else. What is matter made of?
I see your overall point and agree with it as applied to this issue, but I kind of think that it is true you can always explain X in terms of something else ~X if you know enough about it. Matter can be explained in terms of mass, for example. Your point is true in that you can’t particularly far without reaching a point of self reference, if you moved on to asking detailed questions about what mass is and what energy is and so on I expect you’d hit some limits, but being able to take at least one step away from “the thing in itself” is crucial if you want to talk about something actually relevant to this interconnected universe we live in.
The key term here is reductively explain. If you have to switch to some other mode of explanation when reduction bottoms out, then reductive explanation has inherent limits that aren’t brought out in the chearleeding rhetoric. On the other hand, all is not lost.