Last we heard, Elon Musk and Amber Heard were "not back together" even though they kiss goodbye and dance real close ... sorry, we're not buying that now.

Amber and Elon went on a sushi date Monday in WeHo, and looked like the full-blown hand-holding couple that's definitely on again. But that's only because that's exactly what they are -- no matter how many times they try to say they're not reunited.

We broke the story ... Elon and Amber started hanging out again this past fall ... after announcing their split in the summer. Since then, they've smooched and gone dancing together.

If it looks like a reunited duck, walks like a reunited duck ...