That's strange, for me double tap is working quite well on Xperia.

And with Android it's also possible to double tap to sleep/close the screen. I find it very intriguing to maybe have both functions also with Sailfish X. But i fear this will unfortunately not happen, if Jolla is not sure if double tap to wake will be present on Sailfish X.

Also swiping from the sides to wake up the device, as already mentioned, would be great.