I assume that they are asking for 1024×576 because that is the default setup with which their broadcast equipment ingests broadcast-ready footage.
720×576, 16:9 is anamorphic and works fine as a distribution aspect ratio. Again, different strokes for different equipment.
I would also suggest that you think about working strictly in square-pixel aspect ratio as not all pugins work well in non-square comps/layers. In fact, with AE’s 3D render engine, you may notice slight abberation’s when trying to finesse non-square layers.
When 16:9 is sent out for broadcast, it is in effect, 1024, where it is anamorphic 16:9 or 1024×576. If it was 720 in both cases, then there wouldn’t be a widescreen effect unless the 576 was reduced. But that just doesn’t make any engineering or aesthetical sense.
Cheers
Roland Kahlenberg
https://www.broadcastGEMs.com – Adobe After Effects project files
https://www.myspace.com/rorkrgbspace