Why PU currents are equal to PU powers with DCPF assumptions?
The assumptions of DC power flow analysis are:

Voltage is constant throughout the network.
Transmission losses are assumed to be negligible.
Power factor is assumed to be compensated at the load such that it is 1.0 throughout the network.

Under those assumptions, per-unit power is equal to per-unit current because voltage is equal to 1 PU and power equals voltage multiplied by current.
Those assumptions limit the usefulness of DCPF analysis, but in is still useful for certain purposes, particularly grid system level economic analysis and resource allocation management.