Their hardware was essentially fixed, well defined, and limited in scope.

They reviewed every single piece of code many, many times.

They were draconian about memory handling, loops, preprocessor directives, and recursion.

All functions had to check all return types from other functions.

No more than one level of pointer dereferencing.

All code bodies had to fit on a piece of paper.

/edit should add, they used their tools thoroughly: compilers were run on max warning levels and absolutely no warnings were accepted, ever. They also used several static analysis tools on the code (under-appreciated these days!).