This was a general description of Product Management, where there is often a single PM responsible for a product or major sections of a product.

Product Management is about more than just implementing specific functionality. At lest that's what it should be. So if you are only responsible for a narrow functional area, then maybe you don't need to know market trends, but then, someone higher up should. i.e. the product is not simply addressing specific functional needs, but is something that reflects what's happening in the market and addressing emergent needs or addressing issues in the market that impact your customers - positively or negatively. It could be regulatory or technological or political, or shifts in market needs etc.