As far as the Panasonic, or indeed the entire L-mount, lineup goes, it really is as lightweight as it gets at 50mm. 300 grams is the same as the Canon 35/1.8 which is, whatever way you slice it, a very compact lens. 
But thxbb12 is right, what the marketing people write is largely irrelevant. They might've never held the lens in their hands, anyway.
But thxbb12 is right, what the marketing people write is largely irrelevant. They might've never held the lens in their hands, anyway.