This somewhat-curious style of programming is partly intended to simplify the task of porting the interpreter to other (perhaps radically different) execution environments.   The programmers work in the “language” that is defined by those macros, so that they can ignore, insofar as possible, how this-or-that macro will expand on any particular computer.

It is “an acquired taste.”

And it runs like the Devil’s own (furry, winged) bat.