A digital thread focuses on the value stream describing the journey of the product from the concept to the salvage yard. A digital twin focuses on the actual subject like a product or a plant.
The digital thread can compare the correlation between the original design intent and the as-built product as it can potentially have component level traceability of the path between the two. The digital twin on the other hand helps take the current state and simulate it for different inputs and arrive at scenarios for possible outputs.
Both technologies have common aspects like modelling, data handling, visualization and other components that make them deliver what they do. But a digital twin is different from a digital thread as it also has the ability to simulate scenarios. Digital thread is different in that it can establish the genealogy of information across the lifecycle.
Although the two have their different purposes, a digital thread is a key component to building an accurate and precise digital twin.