I think the logic is to make if a "primitive macro," the way + is a primitive function. Just as you can't define + in terms of Arc functions, you can't define if in terms of Arc macros. Nevertheless, (isa + 'fn), and so the thinking is that (isa if 'mac) makes sense.
There are valid reasons not to do things this way, of course. For instance, what's so powerful about macros is that they literally expand into a code tree, which (as you observed) if can't do. For that reason, why not have (isa if 'form) (or 'prim, or 'special, or something similarly descriptive)? Then you have first-class special forms without literally making them macros. This would be a big improvement over the current state of affairs:
arc> (type if)
Error: "reference to undefined identifier: _if"
.
