The problem is the graphics card of the small MBP. Here you only have an Intel IRIS 655, which supports max. Displayport 1.2 for video output and not 1.4, which you need for the Dell. Someone has explained it in more detail here: https://www.dell.com/community...
The maximum resolution is only possible if the display can receive two DP 1.2 signals from the MacBook instead of the required 1.4. Dell may still have a current firmware that allows this, but probably not.