I think a manager for a product would give as much work and responsibility to one person as possible if they say they can do it, and sometimes even when they say they don't know if they can do it but they'll try. An experienced manager might know how realistic the workload is and downsides of having only one person on task x but every manager sees the upside, fewer people = less cost.

It makes me uncomfortable as well.

But keep in mind that at that time, it may have well been the only programmer on the project, and that the manager was not likely qualified to read code.