A vector illustration of a red driverless sports car was refused by Adobe Stock because of an intellectual property violation.

They were not specific. There are only two reasons I can think.

1. Driverless technology is owned by a company, maybe Tesla.
2. Red color sports car is owned by sports car company, probably by Ferrari.

Its more likely that the vector design was too similar to an existing actual vehicle design

Agree. Reviewer probably thought it could be too similar to a real sports car or didn't want to risk it.