The genius of this scheme is that TATP is relatively easy to detonate. But you must make enough of it to crash the plane, and you must make it with care to assure potency. One needs quality stuff to commit "mass murder on an unimaginable scale," as Deputy Police Commissioner Paul Stephenson put it. While it's true that a slapdash concoction will explode, it's unlikely to do more than blow out a few windows. At best, an infidel or two might be killed by the blast, and one or two others by flying debris as the cabin suddenly depressurizes, but that's about all you're likely to manage under the most favorable conditions possible.
So the fabled binary liquid explosive - that is, the sudden mixing of hydrogen peroxide and acetone with sulfuric acid to create a plane-killing explosion, is out of the question. Meanwhile, making TATP ahead of time carries a risk that the mission will fail due to premature detonation, although it is the only plausible approach."An infidel or two..."! Gotta love scientific analysis with a touch of snark.
Certainly, if we can imagine a group of jihadists smuggling the necessary chemicals and equipment on board, and cooking up TATP in the lavatory, then we've passed from the realm of action blockbusters to that of situation comedy.