where is your source. How do you know that china is NOT doing that? You have no idea. Just as we have no idea that the US was building a human tracking system since 9/11 and kept it secret during the wars. Now we know.
Beyond that, the US and Russia don't seem to be hiding their work on it, so why would China, which has even less of an incentive, be hiding it?
So the issue is really your premises about it making you "nearly unstoppable", it leads to no such thing. The advantage is much slimmer than you might think, especially right now. As it is, unmanned but non-autonomous weapons provide 99% of the functionality.
Additionally, neither the US, Israel, the UK, or Russia are making their autonomous weapons programs secret.