A) the EmDrive is for real
The article posits a theoretical framework that explains the thrust seen in experiments. That framework basically says that for super-small accelerations, inertia comes in discrete increments. It would follow that the EmDrive will, therefore, never scale up, but that may or may not be a limitation depending on design. I don't understand the framework well enough to have any confidence in opining.
B) It's possibly even better than we thought it could be
Well, again, it seems to me as if the theory put forth basically says "this only works for super-tiny minuscule effects":
At very small accelerations, the wavelengths become so large they can no longer fit in the observable universe. When this happens, inertia can take only certain whole-wavelength values and so jumps from one value to the next. In other words, inertia must quantized at small accelerations.
They do put forth the idea that (1) the thruster can be tuned and reversed (2) efficiency can be increased with a dielectric (TBD) which is interesting. I understand this thing well enough that I know I couldn't build one and I know I wouldn't know what I was looking at if I saw one but I think you could have a whole bunch of them strung together which might defeat some of the limitations. But right now, we're at a tenth of a newton of thrust with 17W of power input and the discussion at hand is "let's explain this anomalous thrust" rather than "whoopee we created an impulse drive."
C) this has the potential to upset our understanding of physics / roil some people's blood
McCulloch’s theory could help to change that, although it is hardly a mainstream idea. It makes two challenging assumptions. The first is that photons have inertial mass. The second is that the speed of light must change within the cavity. That won’t be easy for many theorists to stomach.
So... I dove into the actual paper and this part is troubling:
Normally, of course, photons are not supposed to have inertial mass in this way,
but here this is assumed. It is not clear what the size of this mass is, but it is
clear for example that light inside a mirrored box produces a kind of inertial mass
for the box.
I dunno. I'm not a theoretical physics genius (I'm not even a theoretical physics lunkhead) but the phrasing of that particular condition is troubling. The whole "do photons have inertia" appears, to my lay read, to be very much a semantic discussion that's wholly dependent on how you're doing the math and I'm not near clever enough to argue this one, but this looks like some supreme hand-waving.