Why does $x^{15}+x^{14}+1$ detect all errors at most 32768 bits apart?
Your polynomial is primitive, which means that the order of $x$ modulo your polynomial is exactly $2^{15}-1$. In particular, $x^a \not\equiv 1$ modulo your polynomial for all $1 \leq a \leq 2^{15}-2$, which means that your polynomial doesn't divide $x^a-1$ for this range of $a$.

(This also means that your polynomial should divide $x^{2^{15}-1}-1$, in contrast to what the question claims.)