“Spoon roast” is a term that refers to the recipe or the method of cooking and not the cut of meat. For example, there are many different cuts of meat that are labeled “pot roast” and “London Broil”. The label of these cuts referring to the method they’re cooked i.e. pot roast is cooked in a pot (braised) and the London Broil is broiled under intense heat.
Spoon roast is usually braised (cooked slow in a liquid as you mentioned). The roast could come from a variety of cuts, such as Top sirloin, bottom sirloin, or even the round area. Since the butcher told you that this cut of meat contains a piece of the Tri-Tip, there is only one place it could have came from, and that’s the bottom sirloin. If you can’t find Tri-Tip roast next time, ask for bottom sirloin roast or Sirloin Tip roast. The Sirloin Tip roast is the closest thing to the Tri-Tip. They both contain pieces of each other’s and they’re cooked the same way.
Here is a link for cooking instructions: