I think by high chemistry they mean highly volatile. By low chemistry they mean not volatile.
I do see a lot of people - even BSs here - who take the view that if a couple doesn't fight that they're passionless. Which is weird to me. I don't yell at people, I don't want them yelling at me. I certainly don't reserve my worst behavior for my husband. Why would I treat my co-workers better than I treat him?
So high conflict relationships are probably being confused with chemistry here.
I see chemistry when somebody gets your references and you delight one another.