It sounded incomprehensible to me too but as I did some more learning about the process of building rocket engines I learned some interesting details. First, remember that people have been making high strength metals through careful processing for thousands of years. Second, the parts of engines are not made as part of large-scale industrialized manufacturing. Almost all the parts are made as few-offs, with far more energy, time, and effort put into making sure that a single instance of something is extremely reliable. Third, we got damn good at materials science in the past 100 year, and metals can be absurdly resistant to deformation under heat.