Except it's not so nice, because despite being to a place called Exeggutor Island twice already, Exeggutor still hadn't been seen by Pokedex standards until the Blue battle and is in fact another new entry there, making the real count 170.
This leads into a different matter: Would a playthrough of X, or Omega Ruby, be interesting enough with this same objective in mind? Alolan Dexit runs feature the unique dynamic of SOS battles, such that new Pokemon can appear in the middle of a battle, most notably in the totem battles where they represent the main difficulty gates of the run: if and only if I have someone on the team who can manage to dismiss the totem in a single turn, I get to avoid racking up another Pokedex entry. Other than that, the only difference between this and a regular old low-grinding run (where it's just a matter of "can my team overcome these other teams by some means") comes in the multi battles, if there are any of those where my teammate's backups haven't been seen yet at the time (like in Aether Paradise here, most infamously with Faba 2).
Is there enough left at that point to make it a genuine challenge, you think? Are people just clamoring that much for an index of every battle in the game that features an opposing party of 2+ and that it's okay to lose? Are there still any unelucidated mechanics I haven't yet gone over that have relevant tie-ins to the course of these other games?