I think the commenters are right that there is a separate delay here and it makes sense from thinking about how the firmware works. The button delay parameter is intended to control the ability of the switch to detect multiple presses in rapid succession. It doesn’t seem like it’s intended to control the ability of the switch to detect a held down switch.
Unlike with multiple button presses, there has to be some delay, because otherwise it’s impossible to distinguish between dimming and a button press. This is just like eliminating the button delay takes away the ability to detect multiple button switches, except that the consequences are different and likely almost never wanted (unless you wanted a dimming only switch). It’s not really possible to do completely (0 millisecond) instant dimming as doing so would disable the ability to do button presses. Every button press would count as a brief dimming period. As a result, I think it’s certain that this is not controlled by the button delay parameter, but is likely hard coded.
I don’t know if it’s possible, but it might be nice if this were configurable. I too find that it sometimes takes too long for dimming to start — maybe some people linger on the button when holding it down and so reducing the delay would result in accidental dimming rather than switching? I’m not sure, but I suspect that I might be happier with the ability to decrease the delay in recognizing a held down switch as it seems longer than I would expect