Maybe it was done to match the esthetic of the seat tube cluster. That's (also) part lug, part brazed. You can definitely see some outlines of a lug at the top tube/head tube junction, if you zoom in far enough.
Top tube is oversized isn't it? The normal lug set would be sized for a 1" top tube, that looks to be 1⅛". Similar treatment at the BB as the down and seat tubes are conical so the BB ends wouldn't fit in the sockets of a regular shell.
Work in progress