He didn't actually say they are working on one. He said:
>"It’s funny you should mention that," Musk said when asked about making an aquatic car according to electrek.co. "We do actually have a design for a submarine car, like the one from The Spy Who Loved Me – the Lotus that sort of turns into a sub."
The man that wants to go to mars so built a space company, worried about climate change so made an EV company, got angered by LA traffic so made a drilling company, was worried about AI so made a Brain-computer interface company.
>"It’s funny you should mention that," Musk said when asked about making an aquatic car according to electrek.co. "We do actually have a design for a submarine car, like the one from The Spy Who Loved Me – the Lotus that sort of turns into a sub."
implying someone had done a design at some point in the past (from https://www.esquire.com/uk/latest-news/a27939112/elon-musks-...)