I think by strength people mean her physical strength. I don't think it's a blood limit.
Also, she's by far the most powerful of the sannin, or as a matter of fact of all characters, in terms of physical strength. Jiraiya and Oro own her in all departments except for the physical strength and healing abilities. I bet Jiraiya and Oro don't have enough strength to swing gama-bunta's katana like how tsunade did.