I do have a Logitech mouse and as far as I'm aware all the buttons are customisable (through their questionable program), so hopefully this is just marketing nonsense
Oh, it's definitely marketing nonsense. The question is if they managed to hide a sane, usable product underneath it, or if they've added some kind of anti-user nonsense to prevent you from repurposing the button.
Is logitech still not allowing you full customization with presets saved on mouse itself? I'd expect that from the cheapest noname 5-button mouse on the shelf, not logiteck, I thought they got their shit together already given how much praise they usually get.