It doesn't necessarily mean that all the new Pokemon will be available in the Isshu dex.
Just that there will only be new Pokemon appearing until the end of the main game. It leaves open the possibility for a DP scenario, where Evos and random Legendaries are outside the dex.
Just that there will only be new Pokemon appearing until the end of the main game. It leaves open the possibility for a DP scenario, where Evos and random Legendaries are outside the dex.