Tesla doesn't makes their own casting equipment (yet?). They do custom order the largest presses built by Idra Group in Italy (owned by a Chinese company) though [1].
No - I mean manufacturing. At a certain level of demand it could make sense, especially if the classes of devices you seek to produce are heterogeneous in their engineering - aerospace vs phone vs car.
The other advantage with vertical integration through the manufacturing piece is turn around time on new designs. It is a lot faster/easier to spin new test lots for speculative designs when everyone works for the same org.
You could s/chip/battery/ and make the same incredulous statement 5 years ago
Or upholstery, or charging infrastructure, or casting equipment, etc.
Tesla had the capital to go as vertical as they want.