They're asserting that they *don't* track everything. Metadata is important, but you're not going to roll up a plot hatched by previously unknown participants with it, there's not enough information for that. The metadata can provide connections between known terrorist operatives and previously unknown individuals, however. When those connections are established, then they obtain warrants for wiretap/content.
Point being, everyone is suggesting that they are operating a panopticon except for the NSA itself. And now you're suggesting that they have failed to stop domestic plots by using capabilities that they don't even admit to having. Next, someone is going to ask why the NSA hasn't proactively stopped all crime based on their omniscience.