Is $CAPP \in P$ known to collapse some quantum complexity classes to classical ones?
This is a bit of a subtle point, but the reason that solving CAPP (and hence finding an approximation to $gap(f)$ with the corresponding level of accuracy) doesn't immediately give a good algorithm for approximating $\langle 0 | C | 0 \rangle$ for arbitrary quantum circuits $C$ is because of normalisation. This is discussed in Proposition 8 of the paper you mention.

But in particular, we don't have $\langle 0 | C | 0 \rangle = gap(f) / 2^n$, where $n$ is the number of variables in the polynomial; we have $\langle 0 | C | 0 \rangle = gap(f) / 2^{n-h/2}$, where $h$ is the number of internal Hadamard gates in the circuit (see Proposition 1). So this means that to get a good approximation to $\langle 0 | C | 0 \rangle$ (say, up to an additive error $1/3$, which is BQP-complete), we would need to approximate $gap(f)$ up to additive error $1/3 \cdot 2^{n-h/2}$, which can be much smaller if $h$ is large.