I'd have to agree. Below freezing the name of the game is to eliminate drafts and cold spots. A sewn footbox will be better for this. Above freezing the drawcord may prove more versatile, although many people just pull one or both feet out of the footbox in warm weather.
The design of the footbox also plays a huge role. If you just sew through the bottom to seal the footbox, it will not be as warm as a proper baffled construction. Also if you create an extra large last baffle and overstuff it, then using a draw cord can create enough of a puff effect to effectively seal off the end of the footbox as if it were sewn (at a small weight penalty).
If your feet stay as warm as your core during the night it might not make a huge difference either way. A drawcord bottom will be easier to make (especially for a first time project) and if this is for summer or 3 season use, the draft issue is not as much of a concern.
Personally I always have cold feet (and live in AZ where I'm used to 90+ temps at midnight) and prefer a sewn footbox for anything less than 60F. Above that I don't even bother with a bag/quilt.