There is no valid human reason. The difference between Kc4 (mate in 12), and Kc2 (mate in 10) is fairly irrelevant. It simply appended the "?!" based on the algorithm that 10 is slightly worse than 12. Either way, it is, indeed, a simple win from there.
By the way, my plan would be Rd7, then bring the king to h1/g1, and only then, bring the rook over via ...
On Lichess, inaccuracies are denoted ?!, mistakes by ? and blunders by ??. You can click on the `x inaccuracies/x mistakes/ x blunders' to move between these.
The move was inaccurate since it reduced a forced mate from 16 moves (Kc3) to 13 moves. Lichess marks inaccuracies for large checkmate sequences, since humans aren't expected to find these.