I understand your confusion, because it took me a minute to think about it and confirm that this doesn't work. It might be surprising that even a machine with complete information and infinite compute power couldn't pick out the needle; after all, SHA-1 is a 160 bit hash. Are there really 2^160 ≈ 10^48 different texts in English that could reasonably have been written by someone living on Earth (add as much additional metadata as you like)? The answer is yes: there are simply too many needles.

Take your favorite book with 160 or more characters. (Probably a large book, but such a book could certainly exist.) Now, for each character, create version of the book with their name swapped out for exactly one other name which does not already appear in the book. So if a character is named Alex, in some other edition of the book that character is named Jason.

Now, consider how many editions of the book there are. There are two combinations (for each character) that are exactly alike other than that character's name being swapped out. So there are 2×2 = 4 combinations where two characters' names are swapped. And 2×2×2 = 8 combinations where 3 characters' names are swapped. If you think about how many versions of the book there are in total, it's 2^C, where C is the number of characters whose names can be swapped for only one other name.

In other words, it's guaranteed that you can generate SHA-1 collisions using one book alone, using only entirely reasonable alternative names for its characters.

Too many needles.