I expect it comes from reactions to earlier attempts to pass US-made sausages for actual mortadella (aka ”bologna"), as in "that's not Bologna, that's <angliciz

I expect it comes from reactions to earlier attempts to pass US-made sausages for actual mortadella (aka ”bologna"), as in "that's not Bologna, that's <anglicized distortion of the name>!", which was then shortened to "that's baloney".