Elastic promises "open"—delivers proprietary

"Entire Work" and "Corresponding Source" are two different things.

If part of the "Entire Work" is derivative of GPL, then, as a *single derivative item*, it can be licenced under the GPL.

But if the "Corresponding Source" contains large chunks of my MIT-licenced work, you can NOT licence the corresponding source as GPL because I didn't give you permission to relicence my work. What you CAN do is distribute the entire corresponding source AS IF it was GPL.

Note that the terms "Entire Work" and "Corresponding Source" imply that the two are completely separate items (linked by the fact that if you pass the "corresponding source" through a compiler/linker, you get the "entire work" as said separate item).

Cheers,
Wol