Every electron is the same as every other electron, as far as we know. Same goes for protons and neutrons, and all other particles, including the force particles like the photon.
What distinguishes matter, and energy, is the arrangements of the particles.
Theories are mathematical relationships used to predict the arrangements from initial and boundary conditions.
In this context, I cannot understand the statement:
***A "theory of everything" is a contradiction in terms? ***
In this context, things are particles, not theories. There seems to be a confusion between theory and matter/energy.
I admit that the universe seems to be guided by theory. Perhaps within dark matter or dark energy there are a static set of yet different (super) particles whose arrangements contain all the known physical laws, and these particles then guide the physical particle dynamics. So the super particle arrangements could be static while controlling the dynaimcs of the physical particles. However, that is pure speculation.
But in a sense a super theory of everything could include the static arrangement of the super particles. So in this sense it is a theory of a theory without being a contradiction. (e.g., GR is a theory of Newtonian theory). I might further suggest that the control is via entanglement from the 16 compactified dimensions of dark energy in a 26-D universe. Still more speculation.
But the point is that mathematics is not reality. Theory is not a thing except in human minds and as arrangements of pencil markings on paper or electrons in computers which correspond to the equations of natural laws.
A theory could be a thing only in the sense of super particles controlling the laws of nature. These super particles would be an aspect if what is commonly referred to as God.
The present quest for a theory of everything is just to get the equations corresponding to the laws of physical particles. We might then go on to a super theory of every thing to determine how the universe implements these equations.