He was an excellent mechanic and I can never remember him damaging anything he worked on.
My Boss never had a problem with the things that he did because he knew not to over do things. The key reason he got away with using the starting fluid is the someone else cranked the engine while he sprayed. This insured that the minimum possible amount was used. He also made sure the battery/ies were charged.
If you do this by yourself it is difficult to judge how much starting fluid to spray in. You spay a bunch in and run to start the car. What happens next is a large concentration and volume of evaporated starting fluid is waiting in your intake to get suck in all at one time. Boom;maybe!
Also these were Cummings and Detroit diesel truck engines with thick piston rings and direct injection (Detroit 71 series diesels have steel piston heads).