It can’t inject anything into the ISO files at rest without bricking then, and I don’t know if an OS that doesn’t verify it’s own image before booting.
As far as I can tell, this is not talking about ISOs installed using Ventoy, but precompiled blobs of things like Busybox that are included in the Ventoy install package. It’s an important distinction. The developer could bundle a tampered blob, include in the documentation the checksum that matches that blob, and then if someone checks with the upstream project and calls them out, say something along the lines of, “Oh, they must have withdrawn that release,” and replace it with an untampered blob. If they don’t fight to preserve the tampered blob, they might even get away with it.
Even if the original issue had anything to do with ISOs, he’s way overestimating the level of protection on many install ISOs, in my experience—they’re just disk images, and presumably all the files read off them are passing through Ventoy itself. Even if you find one that performs some kind of verification, easy enough to change a jump instruction to a no-op somewhere in the machine code guts of a file as it passes through Ventoy, and prevent the verification from executing. (The difficult part is figuring out which instruction to change, but people have done it before.)