The theory was that apple might revoke their ability to use the developer tools/SDKs (Broadly, this is what was stated in the letter saying their developer acco

The theory was that apple might revoke their ability to use the developer tools/SDKs (Broadly, this is what was stated in the letter saying their developer account would be revoked, even though you don’t need a paid developer account to download some of that stuff). Xcode, the SDKs, etc. are licensed, not open source, and probably contain clauses allowing apple to terminate the license.