Aren't all the grids in US/Canada also connected? My understanding was that they are all connected and that the grandparent visualization was showing the frequency differences across major portions of the US/Canada grid. Or am I getting this wrong?
No, they are not. There are four major grids: West, East, Quebec and Texas. Each one is referred to as an "interconnection". Synchronism is maintained within each interconnection, and they may have non-synchronous (i.e. DC) links between each other. Here is a map: