Based on your prior question on another case, you seem to be "obsessed" with sunk cost.
I'd advise you to get off this thinking. You're recruiting for a consulting role, not an economics/research assistant role.
Sunk cost is a useful concept to know in life, but they're not particularly common in cases!
All that said, the case explicity states "The total investment has to consider both, i.e. the acquisition cost of the island should not be considered as sunk." As such, you just need to take the interviewer at their word...especially given that ROI in business still needs to be calculated off all total investments (sunk cost or not)