Not to diminish your shock, but this has been a common criticism leveled against them for quite some time now (I assume since the beginning of their career, but I wasn’t around for that.)
Yes, they borrowed the feel a lot of songs, and outright ripped off others. I think Jimmy Page playing dumb and saying he never heard “Dazed and Confused” by Jimmy Holmes and then happens to come up with pretty much exactly the same song with the same hook is complete bullshit.
I’m pretty forgiving when it comes to accusations of plagiarism in the musical world, because I know how easy it can be to subconsciously internalize another tune and then have it creep up in your compositions. Also, there’s much to be said for coincidence. Melodies and chord progressions will repeat themselves just by coincidence eventually. I’ve heard music on the radio where significant parts of the song remind me of something my brother or I or some friends composed or noodled around with years ago, and there’s no chance it could have been ripped off.
That said, to me, it’s clear that Led Zeppelin stole some (well, many) compositions without crediting the original artist. I’m not sure what the culture was at the time (borrowing and reworking songs has a long history in music) but what they did is certainly unacceptable given the standards of today (and, I’m guessing, back when they were writing and recording, too, but I don’t want to make any definitive statements.) But I still love the music.