The bookshelf has a plausible explanation based on the final scene, but the wormhole has no such explanation from what I recall.

Remember, they built up the 'they/them' notion but ultimately went the 'it was us all along' route, which while now seems much more plausible than I initially thought it was, still remains an unimaginative and rather boring concept after the buildup of "they".

So, the explanation left is that "they" actually did put the wormhole there, but it rings hollow after the "it was us the whole time!" let down. The humans could not have placed it there since they would have needed it in order for humans to arrive at plans A or B to begin with. Even if we allow for the eventual access to physical dimensions of time space, humans still needed the wormhole to start the process.

If "they" put the wormhole there, that's good and well but it's just flat out lazy story telling. If humans did, it doesn't really fit the order of operations, so to speak, and that's what leads us to "they" or "god", which leads right back to lazy storytelling.