Technically speaking, that \(-8^\circ\) is supposed to be kept negative. The negative is a consequence of how the projectile lands during its downwards journey, and hence its inclination at landing should be somewhere below the horizontal, and not above.
Perhaps, in the Cambridge textbook, what the authors were thinking is that if we just say angle, we mean its magnitude only (especially since the angle is obviously negative in such a scenario). Whereas when they say "angle of inclination", an inclination angle can only be positive when the inclination reflects something above the horizontal, and hence should be treated as a signed quantity.
In the HSC, either such discrepancy would not exist, or they have to accept any conclusion provided the working out is correct.