pcoz –
Chazal are full of statements which say that the kidneys in particular are a source of chochma. In fact, the Gemara in Chullin (11a) seems to understand the biblical word ??? (atzeh) as referring to the kidneys, because the kidneys give “eitza.” The Midrash you cite is understandable in that context. It is hard to believe that the term “boich svara” or ???? ???? has anything to do with that, first because the stomach is not the kidneys, and second because this term has always been used as a put-down.