It looks like nonsense because it treats photons as if they were Newtonian particles and with ignorance of Maxwell's equations and relativity.
Start with section 2. It treats photons as particles with some momentum m*v. I mean, what? That's just wrong. Photons are relativistic p = E/c and quantum mechanical, E = 2\pi hbar f.
I mean take a look at this:
"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. "
So in orthodox physics, photons are not supposed to have inertial mass, but also in orthodox physics light makes inertial mass and it's clear that it's so.
The second statement, about light inside a mirrored box, is so because of relativity and the assertion of the equivalence principle. Electromagnetic fields are part of the stress energy tensor (following Maxwell) which feeds into the source term of general relativity. So yes, there is some sort of inertial contribution, but in fact it can be computed pretty exactly, and it's extraordinarily tiny, and really mostly related to the energy density of the EM field.
So relativity sometimes, but not other times? WTF?
And if the non-standard theory that inertia comes from matter interacting with Unruh radiation, how exactly does that work with photons? Photons don't interact with photons. Zero cross section until the point that they are so energetic they can pop out electron/positron pairs from the vacuum, which is so far not an experimentally accessible regime.
Presumably the idea is that the Unruh radiation inside the cavity is quantized in a particular way different from free space, but wouldn't that mean that inertia of (presumably charged) particles inside that cavity would be altered? But he was talking about the non-sensical 'inertial mass' of the photons themselves. WTF?
I don't mind non-standard theories and their exploration at all, but it's necessary to be clear which standard axioms are being rejected and which others are preserved, and follow that consistently. I just saw very unclear physics.