I agree that it's a bit odd with the Supra being a bit old and at the end of its lifespan, but I completely disagree with the idea of the Camry. I reckon for brand image and public opinion (for both Supercars and Toyota each), the Camry would lead to the project being dead on arrival.
The current (Australian) public perception of the Camry is that it is bland, boring, dull, a good car, but incredibly uncool. You have to think of it from a brand perspective, Camry joining Supercars doesn't help Supercars, because a good portion of Supercars fans (talking the casuals), would absolutely laugh at it, because its a Camry, and for Toyota, Supercars isn't at the point that it'd truly change the image of the Camry.
Compare that to the Supra, fantastic public perception, known for being a 'cool' car, will hit the ground running with support. Also a huge chance to bring in interest from car enthusiasts who aren't currently V8 Supercar fans, especially the JDM and modified crowd, which in Australia is massive, but also doesn't currently have a particularly big crossover with Supercars. Everyone in those circles know what V8 Supercars is, so the Supra will be huge for them.
Toyota joining with the Supra is bigger than what Nissan, Mercedes and Volvo combined, and Supercars with Mustangs, Camaros and Supra is so much better for total brand image than Mustang, Camaro and Camry. The general public will immediately spot the odd one out.