They are different senses of the terms. In "copyright infringement isn't theft", "theft" is in the sense if car theft. In the title, "stolen" is in the sense of a stolen idea.

Copyright infringement, while it may be wrong, truely isn't akin to car theft. It is however akin to a stolen idea. A car theft deprives the rightful owner of the car, but they don't otherwise care that the thief now has a car. An idea theft doesn't deprive the thinker of the idea, but they care that the thief is benefiting from the idea without compensation. Yet they don't care if someone becomes aware of the idea, but keeps it to themself.