No. The sunk cost fallacy involves sticking with something you know is the worse option (sticking with that option is the worse option) by appealing to the investment already made in that option as a justification. It does not mean that all appeals to investment are fallacious. If I invested 5 years in graduate school, and I'm a few months away from receiving my doctorate, it would not be categorically fallacious to argue that since I've already invested this much time, I should stick it out even if I'm thinking of pivoting. You would be losing a degree for what could be a vanishingly small benefit of leaving.

In line with that, the comment above is asking for some reason that could explain why someone who has invested so much time in something would leave so abruptly. A person who has worked on a project for so long derives (and provides) many benefits for having been involved for so long, and even when people change course, they don't typically do so this abruptly without a proportional reason. Hence, the mystery.