Is the Pentax KP's built-in flash only half as powerful as that of the K-7?
Yes, the Pentax KP built-in flash Guide Number is only about half the Pentax K-7.
Page 128 of the Pentax KP user manual specifies the Guide Number at 6 meters.
Page 312 of the Pentax K-7 user manual specifies the guide number at 13 meters.
There is some speculation in online Pentax forums that the KP’s better ISO performance can make up for the weak onboard flash. Others say it is really to only be used as a wireless controller for off-camera flash.
In any case it is a big departure from all other Pentax DSLRs and also DSLRs of all manufacturers which generally have a Guide Number of about 13 meters.