I have personally talked to one of those undecided voters, who is not sure whether they are going to go out on the day of the election and cast their vote. Mostly,.it seems to happen in states which are already irrelevant for the result of the election, for example California, Delaware, Texas, those States which are already deep blue or deep red and voters whether whether it is worth it to go outside of their homes that day because they already know what the likely result for their State is going to be anyways.
Though, undecided voters in swing states like Georgia and Nevada are a completely different thing and that is the population both Kamala Harris and Donald Trump are aiming to.
Those voters will decide what to do based on the incoming October surprise.
Have those people you spoke to registered to vote? I don't know how it is in your country, in my country you don't have to register, but in the USA you do. If you don't register in advance you can't vote on election day. Anyway, as you say, their vote doesn't seem to make any difference as they are not in a swing state, that's why I was wondering. I think in that same situation of doubt I probably wouldn't even register. Just as I think that in the main battleground states the register vote will be at an all-time high.