Yes, it's a little bit both. Some come from lesser known academic journals and some are written specifically for the test. The original works are formatted and edited for clarity so that RC questions with absolute right/wrong answers can be constructed.
Much of the material is taken from published works (hence the attributions at the end of every preptest), but it is abridged and edited specially for the LSAT. I think that's what @jkatz1488 meant.
It's terrible waiting! I keep checking my email for the score, get paranoid the email went to a spam folder, and then just keep logging on to the LSAC website to see. The struggle is real