Initially quite baffling, but then explained to me from the perspective of an industrial designer:
Moses reverses the order that God told him to build things in. God says make a tabernacle, make an ark, make the vessels. Moses says to Bezalel: make an ark, make the vessel and make the tabernacle. Bezalel says to Moses - Might not God have said to make things the other way round? How can you make vessels without a container to put them in? Moses says, perhaps you were in the shadow of God and knew.
After talking to an industrial designer used to dealing with orders that often need further discussion - a compelling explanation:
Moses reverses the order that God told him to build things in. God had said - make a tabernacle (first), make an ark(second), make the vessels (third). But Moses says to Bezalel: make an ark (first not second), make the vessel (second not third) and make the tabernacle (third not first). (This is because Moses assumes responsibility for the whole Mitzvah and is asking Bezalel for the individual parts so that he can create the whole himself). Bezalel says to Moses - Might not God have said to make things the other way round? How can you make vessels without a container to put them in? (showing Moses that he too has a grasp of the whole). Moses says, perhaps you were in the shadow of God (Betzal e-l) and knew (saw the whole picture).
Saturday night notes: Perhaps also thinking about the story in the Talmud this way gives us two hints on why the discussion of Moses' instructions to Bezalel are located right before the story of the Golden Calf. There is a double parallel - on the one hand Moses has to trust someone else to run the show - first Bezalel will be taking care of the tabernacle, ark and vessels, and with the Golden Calf story it is Aaron who is left in charge of the people while Moses goes up the mountain. And on the other hand, whatever it was specifically that the people do wrong (lengthy discussion on whether it was idolatry or not amongst biblical commentators), what is not beyond dispute is that a physical object was created that was described by the Torah as a Great Sin. The start of the parsha also has physical objects that are that are Divinely sanctioned. If this is indeed one of the reasons why the stories of the Golden Calf and of Moses' instructions to Bezalel are next to each other, it is also interesting to think about what it is that makes the two types of objects different. Of course, God commands the one and not the other, but on another level, the vessels, ark and tabernacle all have a utility, whereas the Golden Calf didn't, it was just a symbol. Perhaps that is also a hint towards how to serve God - to look for His help in daily activities and not to try and pin down theological concepts that describe Him.
Final comment on Aaron's statement to Moses: "then I cast it into the fire, and there came out this calf". At lunch today I said to my host - "The Salmon came out great! And she said - it went in great!". Her point was, it was planned. Aaron might be saying here, this wasn't in my control.