Reuters is reporting it as well but no source details, only says Rivian is announcing. They could have picked it up from one of the EV reporting sites, unfortunately hard to trust that anyone is really verifying sources these days.I'm also curious how Rivian made the "announcement." Clearly a press release, but they did not post the press release on their website with their other releases. I looked in the major wires and did not see anything. Maybe someone savvier can track down the full press release.
Samsung SDI to supply battery cells to EV startup Rivian | Reuters
Most of the other sites I see are referencing Reuters.
Update... This site quotes RJ
Samsung SDI to supply battery cells to EV startup Rivian | Financial Post