Does a Robinson Crusoe economy have a substitution effect and an income effect?
I don't see why equilibrium $MPL(\ell)$  would be fixed.

I think Robinson's income is measured by what he can get using his total time $t = l + \ell$.

Assume that given the original $A$ and total time $t$ his optimal choice was $(c_1,l_1)$. If the technology improves to $A'&gt;A$ he can reach $(c_1,l_1)$
 (or $U(c_1,l_1)$ if you take the Hicksian approach) 
 by only using total time $t' &lt; t$.

Let us denote the optimal consumption bundle given technology $A'$ and time $t'$ by $(c_2,l_2)$. This is the intermediate step separating the income and substitution effects. The difference between $l_2$ and $l_1$ would be the substition effect. The 'income' in the two situations is considered equal as $(c_1,l_1)$  (or $U(c_1,l_1)$ if you take the Hicksian approach)  is barely feasible in both cases. Any difference in leisure consumption is due to the different transformation rate between leisure and consumption.

Let us denote the optimal consumption bundle given technology $A'$ and time $t$ by $(c_3,l_3)$. The difference between $l_3$ and $l_2$ is the income effect. The technology is identical in both cases, so any difference in consumption is due to the difference in 'income'.

Perhaps your instructors are trying to say something about the backward bending labor supply curve. It is difficult to tell from the available information.