Thats his point We can't tell the weight of your penny which is the same problem he presented to you on his gold plated bar which would weigh different than a solid gold bar. You could always mail your cents to someone else to weigh for you if you don't want to buy your own scale.
Yup, that's the point. This is a China made gold-plated bar. Either the dimensions or the weight would have to be different from the real thing. You need the weight.
Why would you drop a coin, that you think could be worth tens of thousands of dollars, on a hard surface?
I wouldn't I never had one that I thought would be worth that. And if it's circulated it's been dropped slot worse than an inch flat to a countertop.
To provide a real answer, I have gone through LOTS of cents by dropping them and separating the copper (ding) from the zinc (thud), but the results are FAR from reliable.