imo he probably just took a load of AMD "source" so when he was asked to do some project he could look back at previous work he had been involved with and use it to give himself a head-start. Pretty common among programmers - he was just dumb because he got caught as he decided to copy it all after he left the company.
It is an unfair competitive advantage when you do something like that, but being as the knowledge is in their head too (just not the nitty gritty detail) it's impossible to *stop* an ex employee of one company taking knowledge to the next one, well unless you could erase their brains.