Tesla on the other hand is rapidly scaling up and constantly changing its assembly line and was doing work out of actual tents until recently (probably now as well).
What works for BMW is unlikely to work for Tesla .
Even if Tesla was stable their manufacturing processes and tech DNA are very different from BMW or other major car makers, more in-house vertical integration and electric driver train vs traditional ecosystem and conventional combustion engine.
AFAIK, Tesla does do that at plants that produce both. There have been no announcements of other models for Berlin, but it'd be shocking if it stays only producing Y. There will b similar flexibility when that changes, possibly for new models.