Elastic promises "open"—delivers proprietary

Ahhhh - looking at 2(b) the source of your confusion is obvious.

It does NOT say "the Corresponding Source must be released under this licence".

It says "the Corresponding Source AS A WHOLE WORK .... EXCLUDING any sections that are identifiable and not derived works" (ie pretty much any source file you have not edited!) And if you edit a source file it becomes a derived work, and you must release your modifications to it "under this licence".

You are making the same mistake I have a habit of making - you are not READING THE FULL TEXT. It's called "taking things out of context" and it leads to exactly this sort of mistake.

Cheers,
Wol