Seibel mentions here that of his Coders at Work interviewees, only Knuth and bradfitz did a lot of reading of other people's code, and later his insight that Knuth's reading style was not passive, and more like a "scientific investigation". This post goes into more detail about a concrete example of "reading", very similar to the one in the post (about the Fortran compiler for Bunker Ramo 300, that he read without knowing the operation codes): this one is about a program where again Knuth reconstructs "a listing of Whirlwind opcodes based on the program listing". I think seeing the actual details of what "reading" code entails—the specific margin notes etc–is very illuminating.
It is also probably no coincidence that Knuth came up with literate programming (not only is he a writer at heart, he was a seasoned reader of other people's code, that too machine code!), and also somewhat addresses the complaints about Knuth's own examples of literate programming seeming unreadable: his threshold for what counts as "readable code" is different; code is not expected to be read like reading a novel but more like (his way of) reading a technical paper.