Ok, it was just a confusion:
1) The remains you wanted your rebuilders to rebuild did not belong to you, they belonged to your ally.
2) Two control toggles were preventing your rebuilder from rebuilding your ally's remains:
- As in your screenshot, your "Engineers Do Not Assist Allies" toggle was on. This prevents your rebuilders or engineers from trying to assist ally stuff. This includes rebuilding.
- But even with that set to allow your rebuilder to work, your ally's "Ally Rebuilders May Rebuild My Units" toggle was not on. This defaults to off because the ally would actually have to pay the m+c for the rebuilding process (where just assisting with an engineer to speed something up uses your m+c for the speeding up). So we allow this kind of cross-rebuilding but it has to be a concious decision on the ally's part to allow you to write checks their stockpiles have to make good on
When I change those two toggles (the first on your controls window, the second on your ally's controls window) the rebuilders get right to work