They are releasing a summer-themed game at the start of summer, its not complicated. The roll out plan is a longevity play, the maps to come in the coming weeks are all done (the are already in the single-player mode for the octoling fights). If they wanted to 'meet the checkbox' on map count for reviewer spreadsheets, they could have. They deemed it counter-productive, though, to the product's long-term successes.
The modes, however, are very likely just not done, in fact the whole bulk of the August update seems more like a response to early criticism, which was simply never going to make it to completion in time for retail which was, more than likely, set and planned for, for a long time.
That said, even if we move beyond the spreadsheet checklist for 'content' (and after many hours of gameplay, I now REALLY disagree with the notion but to each their own, though I'd really like to access Splat Zones tomorrow), I don't know how much higher this game could really go. 8s and 9s are fine and that's the majority of its scores anyway. There's plenty of kinks to be worked out from the game and it doesn't really deserve to be earning 10s across the board or anything like that, its average would be a bit higher without the handful of extremely critical scores... which, for all we know, may not have changed for the better.
Just because it has an 81 average on metacritic isn't going to stop this from being one of the most mechanically impressive and deep games released this generation, and one of the better games of the year (mechanically, this is the best game of the year). Just as how having a 70-something score hasn't prevented Destiny from being fun and enjoyable to play.