The statement that Kyokushin is a mix of goju ryu and shotokan is a simplification
(it can often be so convenient to simplify things
Kyokushin's founder studied shotokan and goju ryu karate.
He then developed a new style, based on and influenced by what he had learnt.
I don't think that the idea or intention was to simply mix the two styles to create a new style.
It was probably more like taking (not borrowing
) what he liked best from each style, and change and improve what he wanted to change and improve.
(but that is just my guess, I can't know what went on in his mind back then
But I don't know which of the two styles had more influence on the kyokushin kihon wasa.
You can keep in mind that shotokan karate and probably goju ryu too (I don't know that much about goju ryu) has developed a lot through the years since then. You can find old footage (youtube) of Funakoshi doing heian and tekki kata and it looks rather different from modern day shotokan.