He means that full reserve is superior to fractional reserve (obviously)
If too many people realize this at once, you can get a bank run because bank credit is based on the bank's loan book, which is largely a mortage book and will not be made whole for many years
This could lead to liquidity crises and, in turn, insolvency crises, needing more bailouts, bail ins, bank runs and printed money -- we see it in some countries already
We've never had a full reserve asset before, and everything else must adjust to that.