If S is telling the 'truth' as he discards, then the T♦ and Q♦ are all he held, then playing hearts is the obvious answer. Why would S 'lie'? Possibly to dissuade W from leading another diamond to a void in E. The A♦ when lead revealed to S that the king wasn't in dummy but 3 small ones were. W's double and lead further suggests that W holds the K♦ and would probably lead it next. And quite possibly another♦ for the road. If W hasn't got the A♥ because S has it, S knows that this will make W reticent about leading a heart. It definitely gives more weight to the 5♠ bid. Even if S is void in ♥ it's the♦ that will bring him down and the T♦ Q♦ discard is all about discouraging the third ♦ lead.
