This means (貴様の発言は)まるで他人事のようだな, or "(Your comment) sounds as if it's (about) someone else's problem (although it actually is not)". Here, MC is saying he himself should be suspected and refused, almost like he isn't interested in defending his own position.

So, interestingly, although the literal meaning of this 他人事だな is "It's someone else's problem", its actual implication here is "It's your problem". 他人事 is commonly used as part of (実際には他人事ではないのに)まるで他人事のようだ, so 他人事だな on its own used like this can have the same implication.

This means (貴様の発言は)まるで他人事のようだな, or "(Your comment) sounds as if it's (about) someone else's problem (although it actually is not)". Here, MC is saying he himself should be suspected and refused, almost like he isn't interested in defending his own position. The leader pointed out such a behavior is unusual.

So, interestingly, although the literal meaning of this 他人事だな is "It's someone else's problem", its actual implication here is "It's your problem". 他人事 is commonly used as part of (実際には他人事ではないのに)まるで他人事のようだ, so just saying 他人事だな can have the same implication.