Partner played this hand the other day (I was South). I've run it through Bridge Baron and 6♥ makes (I also remember seeing 4♥+2 results in the Results section). One of the ways I try to get better at Bridge is to analyze the hands we've played, but I can't seem to find a way scientifically for N-S to make it to 6♥.
In the auction, South bids 1♠ to show 4+ spades and 6+ points; this bid also has the unintended benefit of silencing E-W out of their best fit (spades), but it technically does not confirm support for hearts. Even if South bids 2♥ instead of 1♠ (confirming an 11 card fit) West can then easily overcall 2♠, and E-W can certainly compete all the way to 4♠, disrupting the N-S bidding. (4♠ E-W makes).
Even pretending that West doesn't use the 2♠ overcall, North has very few means to explore for slam because the single raise of South is naturally limiting (we play Bergen Raises). 4NT after the 2♥ raise, asking for key cards, would most certainly be a stretch considering the combined high card points of N-S; and, even if North decided to ask with 4NT, he would learn to his dismay that South had literally zero key cards in his hand. With our omniscience, it is easy to see that the Queen doubleton of South allows N-S to make a slam, but there's no way South can know that such a weak suit holding is actually worthwhile.
If the bidding proceeded 1♥-Pass-2♥-Pass (again assuming West doesn't overcall) North could try a splinter with 4♣, but again South has no controls to bid and must bid 4♥, leaving North with no way to see that slam is possible.
(Interestingly enough, 6♦ N-S also makes, but that has to be impossible to bid, haha).
Am I right in this analysis? If there is something I am missing, please do share! I hope you've enjoyed seeing this rather interesting two suited hand!