I think they're just hedging for the fact that there could theoretically be multiple local maxima, but in practice the current-voltage curves of PV panels are b

I think they're just hedging for the fact that there could theoretically be multiple local maxima, but in practice the current-voltage curves of PV panels are boringly well-behaved and so there's just one maximum. (In electronics if you find complex numbers they're usually being used as shorthand for complex exponentials via a Laplace transform — in this case, an imaginary or complex solution would mean that the PV panel has some kind of weird resonant behavior — again, unlikely to be the case for any panel you actually have.)