1) People have been cloning the repo for a week. That's a pretty trivial search to do on Github/Gitlab/etc.

2) They can print out copies of Reddit, HN, other websites to show people bragging about using Youtube-DL to download music videos to "stick it to the RIAA." They only need to establish a specific number for purposes of statutory damages, if the case goes to trial. Otherwise, the specifics don't matter; only that many people are using the tool to infringe on RIAA-members' works.