If the use of defined on aggregates doesn't match the common expectation; and the actual meaning is non-useful; doesn't it make sense to make it useful by making it match the common expectation?

Meh. I see little advantage to providing a second, more verbose, less clear way of getting the exact same answer. I much prefer a compile-time notification of "you put 'defined' in there, but that doesn't do a dang bit of good; maybe you should double check that code or reread some documentation". This author may have been trying for some other meaning, even if such is not the majority case. Why leave a bunch of people scratching their heads about why some places use 'defined' and some don't?

But, even if such were the chosen design decision, it still would have required that first, the old meaning be deprecated. So the current state is required no matter what.