Success needs great vision of what can be in the near future, and carefully pushing that as far as possible but not letting "feature bloat" push it too far out. You can find this vision in both marketing and in engineering teams. If you are great at engineering (likely if you are reading this here), then you can hire people in marketing to sell whatever you create; while if you are great in marketing you can hire engineers to create your vision. Either works.
That the vision seems to have been in marketing in Sega's case shouldn't take away from all the engineering effort that went in as well, but the vision, and thus credit, is on marketing making the right decisions. There are plenty of other engineering companies that likewise have a good vision.
The argument is that Sega's global success with the Genesis came from its products - not the marketing (despite what the marketers say). One piece of support for that is that the console succeeded against Nintendo in most territories, despite all being under different marketing leadership and following different strategies.
When the Sega of America marketers were put in charge of product development and stopped relying on Japanese products, they were unable to replicate their success. They followed questionable practices derived from the toy industry (from where CEO Tom Kalinske emerged), such as a heavy emphasis on expensive licensed titles over original franchises.
That the vision seems to have been in marketing in Sega's case shouldn't take away from all the engineering effort that went in as well, but the vision, and thus credit, is on marketing making the right decisions. There are plenty of other engineering companies that likewise have a good vision.