The shell is closely tied to running commands and the exit code of commands make up the true/false values. "return 0" in the main function of a C program means "no error" which is interpreted as true by the shell. Different non-zero return values signal different command (program) specific error states.

"The shell is closely tied to running commands..."

s/commands/programs/

But yes, I agree.

To arrive at a return value, APL evaluates a mathematical operation, whereas the shell evaluates the result(s) of executing a file.