Proving what? May day was (and is) celebrated worldwide. It has its origins as an iron age European spring festival marking the midpoint between the solstice and equinox. Sometime in the late 19th century it got an International Worker’s Day designation added to it. It’s always, to this day, huge in Germany independent of its brief dalliance with communism. It’s like saying Russians and Germans were allies because they both celebrate Christmas.
Why would German officers be saluting Red Army officers six weeks before Barbarossa?