Well, you're wrong about the constancy of isotope ratios. There're plenty of processes, chemical, biological, and physical that lead to isotopic fractionation. The question is whether bananas enrich the radioactive isotope of potassium in the fruit. I don't know whether that is the case for bananas, but it is so for other fruit.
Wikipedia often provides an incomplete picture. You might want to read other sources before using all that bold text on uninformed shit.