How to differentiate the product signal $f(t)\theta(t)$, where $\theta(t)$ is Heaviside's unit step function?
The answer has already been given in Fat32's comment: if the solution manual says what you claim then it is wrong and you are right.

What I want to add here is that the product rule for distributions is by no means some "engineering heuristic" or some dubious magic, but it can be proved in a rigorous way (see e.g. this document, p. 5) by treating the combined function $f(t)\theta(t)$ as a distribution. I point this out because you don't seem to be sure about this (you mention: kind of "product rule", "product rule" heuristic).

Furthermore, also the fact that the (generalized) derivative of the step function equals the Dirac delta impulse can be shown in a rigorous way. So even if we use notation that seems appalling to most mathematicians, we engineers tread on pretty safe ground when it comes to the use of distributions as given in your example.

A very good introduction to distributions and singularity functions that is rigorous yet accessible for engineers can be found in The Fourier Integral and Its Applications by A. Papoulis.