Borax was a common cure for salmon or steelhead roe in Southern Oregon when I was growing up. The likeness to soap/detergent may have been beneficial. If you take a chunk of that roe and put it in the water it will give off a whitish/milky looking cloud.
Some of the guides claimed that this smelled (to the fish) like detergent and you could excite them in a hole by "accidentally" putting some liquid dish detergent into the water up-stream of where you were going to fish.
I'm happy to say that I can't tell you if it worked or not as the practice of chumming was illegal in most Oregon waters even all those years ago.
As for how the guides knew or believed they knew that it worked...I can only speculate.
BTW, the guides I heard discussing it are all dead now, so I guess the statute of limitations is probably lone past