It would help if you watched the original video. His concern is that the lending halves the value of the gold. My correction is that the bank has acted as a middleman so whatever else you worry about, having the value of gold halved is not one of them.
As you say, this is elemental stuff.
When the bank issues certificates that have no backing, they are in effect counterfeiting the gold. This "lowers the value of money" (ie. causes inflation) since suddenly the market thinks there is more gold available than there really is.
Your characterization of banking (ie. being the middleman between savers and borrowers) is the ideal of how banking *should* be in an honest system.....but it has little relation to how banking actually works today. A bank today is not really a middleman, its simply a money printing machine that creates money upon signature obligation.
Even the money that make deposits, are actually just money created from some previous loan.