None of which makes the game not their property. It's their right to prevent people from doing this if they want. I don't disagree that they shouldn't, but ownership doesn't hinge on the recency of the work nor the profitability of it.
They could defend, but there is significant precedent to allow decompilation and reverse engineering and even Nintendo won’t go after projects that decompile code, but don’t include original artwork or audio (hence the need to supply your own copy of the game for this port).
