The only way she would come out with that statement is she sees polls indicating voters are moving toward Trump and/or has no confidence that Biden would perform well in a debate with the President.
If Democrats are terrified of the gaffe stricken and possibly dementia-stricken, Joe Biden making a fool of himself during a debate with President they should’ve pushed for a better candidate. If they’re terrified of President Trump looking good and reminding Americans of hos many achievements in the Oval Office while debating Joe Biden–well tough sh*t! The Republicans did pick a better candidate.