Strictly speaking (after observing myself pointing) it's "outside your elbow", right? Which coincides roughly with "width of your shoulders" when your elbow is at your side or your elbow is straight, but can differ by up to the length of your upper arm...
This was the case as far back as the original Mac; I'm not sure which cursors were used on systems before that.
It's instructive to observe the circumstances when the cursor changes: there is some indication of context change, but also the design of the cursor in each context is important.
As discussed in the accepted answer, vertical line cursors predate the slanted shape. Changing to a different style only makes sense as a design decision.
Your pointing/clicking finger will appear to be pointing in an upper-left direction, somewhere between 0' and 45' – exactly like the classic arrow-cursor.
Even if a designer with contemporaneous experience explicitly denied that was n influence in choosing the orientation, I wouldn't believe them. The congruence is too strong.
However, I do think there's something more than "just so" to the darkmighty post you replied to, as well.
Anything that's hand-manipulated, and especially English writing on paper, must face the risk a dominant right-hand will partially obscure it. Thus we've got longstanding design tendencies, echoed on pixel screens, that more primary labels and information tends to be above and/or to the left, and more-ignorable (or even just defer-able) information goes to the bottom/right.
If damaging the visibility of some information for another purpose – as with an overlaid pointing indicator – a designer will thus have good reasons for allocating that damage in the down-right direction. (That is, tend towards obscuring the later/lesser information.) People immersed in our culture's visual design will even make that tradeoff subconsciously. So a "this makes intuitive sense" story, when reasoning about long-ago design decisions, is perfectly worthy of mention alongside other "historical evidence".