The project manager worked closely with the architect, but the architect focused exclusively on the high-level code/engineering part, not the end-to-end understanding of the whole thing that involved management, partners, corporate processes, approvals, etc.
(And note the distinction between project manager and product manager, two entirely different jobs. Project managers stereotypically use Gantt charts to focus on timelines and contingencies and processes; product managers focus more on user needs and product-market fit.)