See for example the UCL small tutorial in gamma-gamma physics. It says this: “From Quantum Electro Dynamics (QED) we know that photons cannot couple directly to each other, since they don’t carry charge, but they can interact through higher order processes: a photon can, within the bounds of the uncertainty principle, fluctuate into a charged fermion/anti-fermion pair, to either of which the other photon can couple”.
Photons are two rotation particles but charge coupling needs 4 rotations. As soon as two photons meet on a dense matter surface able to carry additional magnetic flux (Holmlid case) the "photons" potentially can interact.
It's all mater of definition: Photons produce "virtual charge" because they are made of magnetic flux. But virtual charge does not directly interact with "real" 4 rotation charge.