Huh. I've been using a Clara BW since it was launched a couple of years ago, and never noticed this function. I've always, and only ever, tapped the centre of the screen to get the reading menu.
Anyway, it might be worth your while asking in the
kobopatch request thread if this can be patched. (I don't see anything in the currently available patches that'll help, but it's not impossible that I've missed it.)