Indeed a sync'd grid is what is typically being called a single grid. HVDC only got the required fast breaker a decade ago so it is nowhere near as widely deplo

Indeed a sync'd grid is what is typically being called a single grid. HVDC only got the required fast breaker a decade ago so it is nowhere near as widely deployed as the AC system (it's hard to make a HVDC breaker because even if an AC breaker arcs it'll self extinguish in an extremely short period of time as the voltage crosses zero, HVDC never does this so you need something to force the current to zero). Maybe the definition will change over time, it is, after all, completely arbitrary.