If you want to split the duties of a tech lead and a PM, say because multiple tech leads are required for a large project, managed by a single PM, then the PM still needs to juggle various customer requests and split them into team-sized requirements for the tech lead to split into developer-sized tasks. That you've introduced a higher level of project abstraction over multiple teams does not change the fundamentals - PMs being intimately familiar with the output of individual teams to identify strengths and weaknesses, ensuring that teams are adequately staffed and qualified, monitoring the backlogs of teams so that the PM does not commit to work which the teams do not have the capacity to achieve.
