How can a TPM be helpful if they are not the most technically advanced person on the team?

In my opinion, you can be successful in any role by not having the expertise if you recognize it and are willing to learn. The product manager is generally not the most technically advanced person on the team, but they bring in other expertise and experience to the team (e.g., customer data, market insights, etc).

What has helped me is identifying key experts in the organization and leaning into them. I've usually not found a single technically "advanced" person on any of my teams. Different members of the team might have expertise in different areas. PMs with the expertise to identify these members and leveraging their knowledge can be successful in any role.

In a word, abstraction. A technical PM is usually NOT the most advanced technical person on a team. However, they are able to break through sufficient tech layers to be able to contextualize the tech in the overall product development. They are able to draw the bigger picture of how components fit together, what technology elements are critical. And how to think about investment in each technical area - sucvh as build/buy/partner. In this way, they're not competing with others on technical know-how, but using it as an essential area of overlap to allow easier communication and collaboration.   

A techical PM is simply able to engage in more meaningful planning, discussions and collaboration on technology areas with other counterparts.