If they were optimizing for gameplay they would sacrifice graphical fidelity until they hit 60fps. Targeting 30fps means they want to sacrifice gameplay for graphical effects.
Not just graphical effects: more NPCs and gameplay entities on screen, better AI, better physics, etc.
In terms of trade-offs, 60fps is pretty much always the first thing to be sacrificed unless it's REALLY important, like in a twitch shooter or racing game.
The truth is that for most games 30fps is not a deal-breaker for the majority of people. If even a significant minority of players cared, the intensely focus-test-led games industry would follow.
That seems to be exactly what they are not optimizing for. Lowering your standards to 30Hz lets you crank graphical fanciness and engine features, but at the expense of gameplay. There is a reason Quake players used to turn their graphics down to mud, even though they could play with better graphics at a lower fps.
Targeting a lower frame rate also allows you to create a world that is mor interactive, a la the quote in the article, by including more game objects and other elements. Graphical quality is still important...hence the lower frame rate.
Those may make the game more enjoyable, and 30Hz may be the correct decision for that reason, but if those things come at the expense of fps they come at the expense of playability.
It is undeniably a trade-off, which way you think is worth it is a subjective matter of course.
