One hand doesn't prove anything, but I agree it's between a spade and a diamond.
On the actual hand, a spade lets it through at once. A diamond gives you a chance. In fact declarer was slightly sloppy and a diamond did even better, because this is the layout:
On a diamond lead, he tried the jack from dummy - queen - king
He played ace and another club. I won and played a second diamond, which he took and ran clubs. He came down to Kxx Kx x - in dummy and Qxx J10x - - in hand
At this point things are slightly delicate. Without considering what might happen next, he played a spade to the queen and ace, then two rounds of diamonds squeezed dummy. But he's in trouble anyway, because the SK off dummy might hold the trick... so he did have to pick the ending whatever he did (or he could have run the jack of hearts earlier)
FrancesHinden writes "IMPs, again. 1NT = weak. Responder has (now) denied a 4-card major, opener could have 4-4 in the majors."
IMO ♦2 = 10, ♠ 3 = 8, ♥6 = 3, ♣K = 1.