(I'm the person who made the camera tools and received their lovely email)
My suspicion is that they think I distribute some of their assets as the page they linked to seems to talk about that. But I do nothing of the sort, I only distribute my own code, none of theirs, no assets of any kind. The tools change code at runtime in memory, which is the same as running MSI Afterburner which hooks into the game for instance. The adjustments are solely done to be able to take screenshots from the game (I reenable 3rd person view too, which is in the game code to begin with, not sure if they're mad about that lol :D ).
What strikes me the most is that they didn't even ask first, like "Hey we saw you distribute a mod for our game, but are you distributing any of our assets?" (which is something I can understand), but they went into full offense mode.
I don't understand what their end goal is, to be honest. I do this now for a long time and I got nothing but support/praise from publishers and devs for my work. My photomode mods are usable in over 350 games (the Unreal Engine Unlocker alone in over 320 games), it's not new or anything :)
We'll see. I've replied back with questions which parts of the EULA I apparently violate (as I'm a EU citizen and EULAs aren't binding) and explained I don't distribute anything from theirs. It's a bit tiring tho.