Are you sure you transcribed the sequence correctly? White 10 as big knight move seems pretty odd and easy to punish: p16 attach looks much better and simpler to me then p15 as it easily connects black and cuts white (whereas p15 cuts white but doesn't connect black so there's a fight, which yes could be a nice fight for black but it's a fight so black has chances to mess up). White more sensible move of small knight for 10 at p15 instead could revert to the shown sequence.
Anyway, regarding q17 vs q18, this is indeed a shape that crops up fairly often. Each has pros and cons. The pros of q17 are that:
1) it takes a white liberty
2) if white answers at p18 descent then if black blocks (it's usual) at q18 then black has 2 more points of eyespace (important) / territory (less important) than compared to q18 first and white pushing at q17, black r17, white p18 black r18. This can often make the difference between a carpenter square type of shape being alive (or at least a seki or weird ko) in sente vs an L type group being unsettled. Getting one extra move in the centre fighting if your corner is already alive is huge.
The cons are:
3) it's an empty triangle so there are some possibilities of liberty shortage if white can get the q18 and s16 points.
4) gives white the choice to q18 instead of p18 to create corner shape weaknesses, though this will mean white takes gote not sente locally.
Pros/cons of q18 are basically the opposite of above.
As for why q17 is bad here, that seems to be because white will indeed play q18 hane, and because black pushed white along on the outside white does have severe aims at s16 if black plays on the top side after p18 connect. And if black defends the corner then white can safely jump to m17, which is not possible without a white stone at q18 in addition to p18 as black has the push and cut tesuji. It doesn't surprise me that a pro like Yilun Yang could get something like this wrong (my suspicion is he didn't give enough thought to white q18 because p18 descend is normally better (for sente) to consider the s16 tesuji followup). There are situations in which q17 is better than q18 though (I've seen Ilya Shikshin play it several times -- he often gets into fights) and plenty of weaker players might not even consider it as rejected by the over-simplistic empty triangle is bad mantra. So it's good to add to your vocabulary, even if it's not appropriate in this book example.
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Proper pincer example 2.sgf [227 Bytes]
