DP v1.4 does support 240Hz but at lower Resolutions.
Found this Cabe Tech article explaining about DP 1.4 and 240Hz Resolutions: https://www.cablematters.com/Blog/DisplayPort/does-displayport-1-4-support-240hz
Note: the above chart shows why when you have your Monitor set for 2560x1440 Resolution, it supports 240Hz.
From Wikipedia showing the maximum Resolutions for 5K DP versions:
EDIT: You will need a DP version 2.0 or above to get 5k with 1440 hz Resolution. DP 1.4 Bandwidth does not support 5k at 1440hz bandwidth.