This description bothers me, because it pattern matches to bad reductionisms, which tend to have the form:

X (which is hard to understand) is really just Y (which we already understand).

A stock criticism of things reduced in this way is this:

If we understand Y so well, why are we still in the dark about X?

So, if ethics is just game theory between agents who share values (which reads to me as 'ethics is game theory'), then why doesn't game theory produce really good answers to otherwise really hard ethical questions? Or does it, and I just haven't noticed? Or am I overestimating how much we understand game theory?