Robert Plant stealing the "squeeze my lemon" line from Robert Johnson is plagiarism, as well as many other documented lifting of riffs and lyrics from other blues standards.
This is inspiration, at most. I've heard before, people saying Stairway was stolen from Spirit. If this is it, I'm not impressed in the least. Then again, I wasn't impressed with the claims that George Harrison stole My Sweet Lord from He's So Fine, but at least that was ever so slightly more convincing.