Is it "common knowledge" that early enrichment can speed up development?
To a certain extent, yes. There's plenty of research showing that early experience is related to how quickly infants learn language. For example:
Of course, there are some natural limits on this. No matter how much high-quality, socially contingent language exposure your child gets, she won't start speaking until she has enough motor control to form the necessary speech sounds or hand movements (in the case of signed languages). There's also a genetic component to intelligence, so some children will naturally learn a bit faster than others regardless of exposure. That said, there is a large body of evidence supporting the idea that infants and children learn faster when they get exposure to lots of high-quality input. It's safe to call that "common knowledge" among language development scientists at this point.
Are the specific details of Fowler's method important?
Although there's lots of evidence that the broad strokes of Fowler's method (i.e. talk to your child) are spot on, the details of his particular program have not been extensively tested or replicated. Most of the studies on this method were conducted by Fowler and his colleagues --- it never got picked up by the rest of the field and widely applied in large, rigorous studies to test its effectiveness.
In terms of evaluating Fowler's own evidence on whether his method is particularly effective, it's really important to know 1) how participants were selected for both the treatment and control groups and 2) what the control group's experience was like. (These issues apply to any proposed child development training program, so these are good general questions to keep in mind when you read evidence like what Medina quoted in the book you have.)
The possibility of participant selection bias
Ideally, participants should be randomly assigned to the treatment vs. control groups (called a randomized controlled trial, RTC). If participants were not randomly assigned, then there could be selection effects driving the difference between groups, rather than any effect of the training program itself. For example, if Fowler recruited parents to take part in the program by advertising for training classes to improve language development and then used families in a local daycare program for the control group, there could be big differences between the treatment groups and control groups even before training started. Parents who responded to advertisements about a training program to accelerate their child's language development might be especially involved or dedicated parents, they might be more intelligent and/or more educated themselves (meaning they might be passing on high IQ genes to their kids already, and highly value their child's cognitive development opportunities), and they might be more likely to also be doing other things already in an attempt to facilitate their child's language development like reading books at home.
What exactly does "control" mean?
Scientists distinguish between "passive" control groups and "active" control groups. If it was a passive control, those parents probably didn't receive any special training at all, they just had their children's vocabulary measured at each time point. In an active control, they would have taken part in some other experience, designed to match the treatment group's experience as closely as possible without providing the actual training. For example, they may have been brought in for sessions just as often as the treatment group was, but instead of receiving the Fowler training they could have gotten general training on child development, or maybe training about motor function, emotional development, etc. Just not the Fowler training. This kind of "active" control is really important because just the act of participating in a program --- any program --- can affect parents' and children's attitudes and behavior; participating in a program can make parents feel more involved, maybe improve confidence and interest in their child's development, etc., all of which might help their child learn faster even if the details of the program itself aren't particularly important. This is similar to a placebo effect often observed in medical studies.
Take home message
There are lots of different recommendations and programs available that purport to facilitate development. Some of them are presented with imperfect or incomplete evidence (as is the case with Fowler's program), and many are presented with no evidence at all. A good idea is to look for points of consensus on general strategies (like the studies I cited in the first section), and figure out something that works for you and your family rather than fixating too closely on the specifics of any given program.
There is no such thing at this point as an accepted gold standard training program to support language development, other than the one we've been using as a species for millennia: You take care of your baby, and he'll pick up what he needs to learn. ;)
