Yes, it is true. It's not an urban legend. Radlers have been around for decades!
It's also not "lemonade", but lemon soda...you know, like Sprite.
The Germans also have a drink called a "Geistmass" and sometimes "Geisermass". "Geist" means "ghost" and a "mass" is a liter mug (the glass ones).
It is half beer and half cola. To which 1 shot of whiskey is added. Man, I've killed many of those when I was younger (read: STUPID!!).