It's based on https://github.com/jquery/jquery/blob/6de1d973a436282af7dfe3072924016c3cdc1984/src/intro.js, but skipping their window concerns. But, to be honest I didn't investigate why Core is following this approach instead of any of the UMDs you pointed out above.

On jquery/qunit#540, where can I find more info regarding "From previous chats with @jdalton, I believe there are a few more variations necessary to thoroughly export for all CommonJS environments, none of which require a nodeType check AFAIR"?