He mentions the importance of automated tested once or twice I think, but it isn't hammered on as a modern Agilist would. But, remember, 1970 was a different world. You may literally have not had room to include unit tests in your code, or literally be unable to afford the sort of abstractions that would enable unit testing, for instance.Other parts of that book talk about the difficulty of squeezing bytes out of pages. A lot of modern Agile practices are not possible back then, or are so different as to be entirely different things. TDD would probably get laughed at by all, for instance. "What, you want me to waste my precious timeshare time to run tests I know are going to fail?"
I don't have the reference, but I read somewhere several years ago when digging through old journals that regular testing was part of the Project Mercury? Gemini? software development process.
I would not be surprised to find that someday we understand why some projects work from agile, and see that it basically comes back to the "vision" observation in Mythical Man Month.
Of course, this really just betrays that I want that to happen. So... take it for what its worth.
It's been a while since I've read it, but I don't think he captured the benefits from agility.