I think that's a typesetting error (which can happen in every good book).
When you assume the same harmonic time dependence for T as for u
(I am skipping the indices here):
dt2 T = - w2 T
and start from eq. (A-87) (independently of the actual form of u) you obtain the denominator of eq. (A-93).
T = -1/(p2-w2) F dt2u
IF the additional w2 in eq. (A-92) was correct, the denominator would be different. From the excerpt you attached, I therefore conclude this is extra term is an error overlooked in the editing process.
eq. (A-93) is the mathematical solution to eq. (A-92) [without the error of an additional w2). You arrive at this solution by "guessing" that T has some amplitude and a harmonic time dependence:
T = T0 sin (wt)
then you insert this and arrive at eq. (A-93).
A physical system that is driven by an external source of a specific time dependence [here sin(wt), eq. (A-89)) typically takes on the same time dependence, hence, this was a good "guess", it gives a valid solution to eq. (A-92).