If immigrants are killing pets to eat because they are hungry then you can be sure they will continue doing so for the next seven weeks. As that happens they will realize Harris is lying about this and Trump was telling the truth. On the other hand if there are not anymore stories about this it will be seen as a gaff for Trump. However, I doubt this will have a bigger impact than what he said about McCain in 2016, so this is simply trivial in the grand scheme of things.
If the economy is strong then Harris will get a boost from that and if that is in fact a lie, then Trump will get a very big boost. When things are good people are complacent, when they are bad they will vote.
What will help Harris the most is if the economy is strong, people are making a lot of money, and all the immigrants are hard workers and not involved in criminal activity. The situation in Aurora Colo., Springfield Oh, El Paso Tx, and Chicago are much more serious for the Harris campaign than whether or not Trump was mistaken about pets being eaten.