I don't think you are missing anything. I think that like most players of the time, Byrne learned to compensate for squirt subconsciously as he was learning to use side spin. Since he did not think about squirt while shooting, it was not important to him.
At the time he wrote SBoP&B I think there was no mention of squirt in any book in print. There is a brief warning in Hoppe's "Billiards As It Should Be Played" (1941) against "spineless" cues without any real explanation of what the result might be if a cue was not sufficiently spineful.
Some pool authors who were probably unaware of squirt are Mosconi, Caras, Knuchell, Cottingham, Lassiter, and Martin. Either they were subconscious compensators or they deliberately left out a phenomenon that will cause most shots with side spin to miss if not taken into account. (Or their ghost authors were poorly informed.)
Interestingly, the first major book in English about billiards/pool -- Edwin Kentfield's "The Game of Billiards" (1839) -- has diagrams showing both squirt and swerve and even has a diagram showing back-hand english (BHE) or aim-and-pivot although the text is not perfectly clear about the technique. A French book from somewhat later (1890?) says explicitly to aim shots without side spin and then pivot around the bridge hand to get the required "effet" which is BHE. In his defense, neither of those books was in Byrne's library.
As for the term "squirt", it seems to have been coined by Jack Leavitt who was an A player in Northern California in the 1970s. I relayed the word to Byrne while he was writing SBoP&B. Coincidentally, Jack sold me a cue at a tournament which I had somehow arrived at without my own cue. It had an unusable amount of squirt. It was a good lesson for me.
As for the physics behind squirt, it seems that was not generally understood until high-speed video was shot of tips on balls, first by Meucci and then by Predator (and the Jacksonville Experiment group). It then became clear that simple conservation of momentum required squirt to exist and that mass reduction would reduce squirt. See Ron Shepard's paper for more info.
There are still lots of people who think squirt doesn't exist and many who will admit that it does exist but is not important to play. On the other hand, Eddie Robin has said that he can deal with any variable in a cue stick (weight, length, tip size) except squirt. For him it is the most important characteristic of a cue. For me, too.
