You are correct. There is no bike company (that I have ever heard of) that has the ability to extrude their own tube sets. The ones who claim "custom-drawn" are real, they simply pay for tubes made to their specifications from whatever supplier they choose based on their criteria (ISO or otherwise). The same is true for all metals (Sandvik, Reynolds -
https://www.reynoldstechnology.biz/ ) and many others. Most bike companies do not want to admit it but their "custom" tube is nothing more than a stock tube butted by the MFG for you, me, them, her, him and any other...