The former seems obviously true, but the latter not so much. Many ruthless leaders (Gates, Jobs to pull the two obvious tech titans) have built sustainable businesses using these practices. It may not jive with the current narrative around performance espoused by HBR, but I'm not aware of any data around this.

Assuming "overworking employees" qualifies you as an asshole (not sure), it's not clear that a culture like that that pushes people to perform almost to their limit is not sustainable.

That would be great if the point of a startup was to be a sustainable business.