A product manager who doesn't understand implementations will not be able to negotiate when an implementation bears on interface design or scheduling decisions. Conflicts arise all the time between implementation, interface and schedule. To resolve them one has to understand all three.

This doesn't mean the product manager must understand code. It means there's a trade-off. If they don't understand software implementation, they need somebody on the team who does (what the article calls a "Technical Product Manager.") Adding another person is possible, but it hikes up the overhead and communication costs while dragging down the speed of development and decision-making.

The best case is a product manager who can understand a given implementation if and when it bears on decisions about what is possible in the interface or the production schedule. That person can make informed decisions because they understand all the factors.

It's fine that not everybody can do all three, but imo we should stay conscious of the trade-off and keep the bar high.