Before disk brakes, and wider tyre options, nobody cared. My track bike has regulations for track, my road racing bike has regulations for road racing. My commuting bike has adaptations for commuting. My gravel bike is any bike I choose to ride on gravel, with or without adaptation. Actually it’s a cyclocross bike. But I’ve ridden my race bike too. Since gravel bikes are basically slack road bikes with granny gears, adding flat bars is the very definition of a hybrid.
Personally I prefer swept back bars to flat. Soma Sparrow are great and I use them on my SS MTB.