That's a classic example. The finger and moon anecdote is taken directly from Buddhism. It's actually really easy to trace the roots of JKD. Some of it is even referenced. It's even easier now that we have web searches. Just take a look at the sidebar of the last JKD article we published Jeet Kune Do Counter Attack Strategies By David Cheng (2007 May/June) and you'll see we did a little back research on Bruce's terminology. Research is not that hard.
Now this is where it gets interesting. I'm not convinced that the compilers of TofJKD were familiar with Lee's sources. They just reorganized his notes. Those notes were not necessarily complete. In the above example, Lee extracted a lot of his combat theory from western fencing. This is even cited in the book. However, western fencing is very specific in its terminology and theory - the product of centuries of evolution - and Lee's notes only touched on the theory, because they were, after all, just notes. A lot of JKD people have taken those 'notes' as gospel. But if they don't understand the underlying theory, then there's a major flaw in their thinking. You can take a look at that article and the sidebar and see if it was an example of just such a situation.