What is correct in this situation?

Your answer is correct. In this situation, you must reduce your speed because you are approaching an intersection where priority rules apply. The traffic sign showing a yellow diamond with a white border indicates that the road you are on has right of way. The truck, coming from the left, has to yield to you because it is on a secondary road (sign with a yield triangle is visible on the right). Therefore, the truck does not have right of way and must wait.

The key point to remember is that the yellow diamond traffic sign indicates priority on the main road. Vehicles on the main road do not have to yield to vehicles coming from secondary roads but should still drive attentively and reduce speed in case of potential conflicts.

So, you need to reduce your speed to approach the intersection safely, and the truck must wait because it is on a road with a yield sign, giving you priority.