The "formula" in the title of the book(s) refers to the method of conversion of lab-determined values into real-world training paces, based on race-proven fitness levels. This is the core value of the book and of Daniels's contribution to distance training (a not insignificant one!). What other coach beside Lydiard can say he/she has produced anything of comparable universal usefulness?
But to create an entire book around this basic innovation entailed including many of Daniels' thoughts about distance training in general, including his various training plans. Without this material, no publisher would have been interested. (When pitching any manuscript for publication, the first question you get is "who is the target readership?" As you can tell from the training plans that were eventually included in the book, Daniels' answer to this question must have been "high school and college coaches and runners").
Is the book itself "dated"? All books eventually sound like the period in which they were written, so yes. But is the "Formula" dated? Of course not. The idea of regulating our various training efforts by pace is now universally accepted, just as is Lydiard's basic insight that general aerobic conditioning is the basis of training for all distances, from 800 to marathon. Most specific contemporary training systems are like footnotes on these basic insights.
But just as not everything in Lydiard's book ( e.g. all the "bounding") is immediately useful to all runners, not everything in Daniels' book works for everyone. It never did; it has nothing to do with Daniels being "dated" or not.
*As an aside, I think a lot of the particulars in both Lydiard and Daniels reflect the specificities of the sport in their respective historical and geographical contexts. For instance, NZL has tons of hills and is geographically isolated (much more so in the 1960s, when travel to and from the island would have been pretty pricey and time consuming). It therefore would have made practical sense for Lydiard to emphasize a lot of hill work and very long periods of "base" training with no racing. We often make virtues out of necessities in this way. In the US, on the other hand, where the sport is predominantly school-based, with year-round competition widely available, it made (and makes) sense for someone like Daniels to recommend training plans that include ways to integrate weekly racing. There is no strict, universal physiological rationale for, in the case of Lydiard, doing so much hill work, or for having such long periods without racing. There is equally nothing physiologically optimal about, in the case of Daniels, trying to find ways to squeeze workouts in around races. This is not to say these things are not useful for some people. It's just to say that, as guides to training, they don't have the same universal and somewhat timeless value of the big innovations that inform them