If there's a city law that says you have to have an environmental impact study for things like this, why wasn't it done in the first place? Was this an oversight on the city's part, or did advocates drop the ball?

Pretty much for any commercial or public development in my town, an environmental impact study has to be done. The law was passed with a bunch of comprehensive zoning not too long ago, and developers and town gov't were extremely surprised when various individuals started demanding the impact studies, slowing down and adding to the cost of planned developments and public projects. It was very much used as a stalling tactic by opponents of such construction to delay and vindictively add cost to a project in hopes of making it unattractive, or scaring off developers in the first place. Also townspeople who had to vote to pay for impact studies before public projects could actually be voted on. Sometimes these were used in conjunction with lawsuits to make it too costly to build, or simply delay until a lawsuit could be filed or decided.

What sounds like a great and needed idea--environmental impact studies--can be misused. Like this guy is doing. And like any number of groups would do if there was a proposed nuke plant on the table. To tell the truth, it's usually the extreme environmentalists in town that take unfair advantage of the process in my neck of the woods, but there have also been cases where they've gotten red in the face and stamped their feet when someone points out that they need to do the same when they plan something big, like an addition to a trail system in protected open space land.

You can fault the guy for his personal views, but not his tactics.