It _could_ be done

The late André Bensoussan worked with me on the Multics [...]. We were working on a major change to the file system [...].

André took on the job of design, implementation, and test of the VTOC manager. He started by sitting at his desk and drawing a lot of diagrams. I was the project coordinator, so I used to drop in on him and ask how things were going. "Still designing," he'd say. He wanted the diagrams to look beautiful and symmetrical as well as capturing all the state information. [...] I was glad when he finally began writing code. He wrote in pencil, at his desk, instead of using a terminal. [...]

Finally André took his neat final pencil copy to a terminal and typed the whole program in. His first compilation attempt failed; he corrected three typos, tried again, and the code compiled. We bound it into the system and tried it out, and it worked the first time.

In fact, the VTOC manager worked perfectly from then on. [...]

How did André do this, with no tool but a pencil?

Those people were programming operating system kernel that supported kernel level multi-threading and SMP (in 60s!), had most features UNIX kernels have now and some that no later operating system has, like transparent HSM. All under hardware constraints that would make modern washing machine to look like a super-computer. Of course they were thinking, then typing.