I don't believe I ever said anything like,
"photons alone could not constitute the mass/energy of a black hole.". And in saying this I am not claiming that they could, or event that what we experience as EM radiation or photons, actually exists within an event horizon. I don't
know anything about the physical reality or composition of what lies inside the event horizon, of what we call a black hole. This is where old maps used to say, "beyond here there be monsters". Today we temper that by saying beyond here {meaning an event horizon}, is the domain of theory, orr in the context of lay discussion very often imagination.
Ask a varied group of physists what E = mc^2 means and how it applies to physics and you will get many different and differing answers. For myself I will once again through out a paraphrased quote from a still up recovered source,
All mass can be thought of as an expression of energy, but not all energy can be thought of as mass.
I don't believe that mass and energy are the same thing, though there is an obviously complex relationship between the two.