If someone rebuilds the engine and tightens down the valve cover bolt too tight (the main bolt actually snapped) could this cause the vehicle to throw a rod?

I just bought a 1996 Civic DX from a private seller. The car is absolutely clean, inside and out. My
neighbor joked that my engine is cleaner than his 3 year old Civic's engine. The seller claims to have
rebuilt the engine (odometer reads 147k) approximately 5000 miles ago. About a week after I bought the
car, it threw a rod pretty violently down and out through the oil pan. The car's fluids had just been
checked, it had plenty of oil- in fact, the oil was as clean as when you pour it out of a new bottle. But I
had noticed that the main valve cover bolt was loose, I wiggled it and it came out in my hand and looked
snapped off at the bottom. Could the seller have damaged something by over-tightening the valve cover
bolts? Or could he have done something incorrectly when he rebuilt the engine? Thank you for you
answers, I appreciate the help!