Algorithm hard is very different from integration hard.

Integration hard: a ton of relatively shallow effort.

Possible to shard out to many people

Often necessary to shard out because there's just too much work to do.

A slog.

Algorithm hard: a small amount of extremely deep effort.

It might take months of study, but then the actual lines of code might be only 100.

It's better to have a single motivated and intelligent person.

More people don't help it go faster, and often make it go slower.

With multiple people to bring along, you need to spend time bringing along the slowest person, which takes significant coordination time.