It was called here on Hacker News by SilverElfin back in early February (4 months back), and I remember because I agreed:
https://www.cnbc.com/2017/12/05/the-case-for-elon-musk-to-co...
https://x.com/Loh/status/362036803517169664
A Wall St analyst was asking questions about it on Tesla earnings calls in 2016 and later wrote a research note about the idea. Been widely discussed since.
https://www.roic.ai/quote/TSLA/transcripts/2016-year/2-quart...
That the earliest mentions I could quickly find.