But isn't this the case with the block files too?
If I opened Bitcoin Core client today and synced it, and I tried to access Bitcoin Cash with the Bitcoin ABC client and I put the blocks folder there in the ABC folder, my blocks would have newer blocks that never happened in the Bitcoin Cash chain.
So I open ABC with these block files and it reaches past the day of the fork... what happens with the existing block files that don't correspond to the BCash chain? It just starts downloading and they get overwritten?
They are validated and ignored. The files are not deleted, they just remain there taking up space. Bitcoin ABC would ignore those blocks (it will validate them, remember their location on disk, and remember their validation status) and continue with download the Bitcoin Cash chain. Because you don't provide the chainstate databases, it will build the chainstate from scratch by going through all of the block files.
Why is this different with the chainstate files.
The chainstate files are for the databases which contain the UTXO set and the validation statuses of every block and the current state of the known blockchain. However when you switch to a different blockchain with the same chainstate database, you may confuse the software because it thinks the blockchain it has is valid (that's what the database says) when it is actually invalid to the software. This could cause lots of problems.