What time did you receive it and did you apply to any other Stanford summer programs?
If you received this email early in the morning then that could be an automated email to all applicants or few who didn’t apply to other available summer programs.
That said, this email was sent to rejected or waitlisted students in the past but only after decisions were released. I don’t know if this is an early indicator though. Not everyone checks their spam folders often so who knows?
Check this old thread for reference.
They use the same admission/application system and email for communication so that makes things easier on their end to target certain students if they chose to.