Electromagnetism is already understood to be a special case of the electroweak interaction, which is itself a component of the standard model. The paper mentions Maxwell's equations, but not these more general models.
So what? The very first step to unifying GR with quantum mechanics and the standard model might actually be to realize that part of the standard model can be embedded in GR (at least when EM is considered as a classical field theory). I don't think it's a deficiency of the paper that it doesn't present a solution to everything.
