Thanks for your thoughtful comment P. Venkat Raman.

In my case it was indeed the best price available. I got the dealership’s best online price, compared it to other dealerships’ offers nearby, and only then negotiated the financing.

Interestingly, they only offered the 0% APR if I financed the full cost of the car. My guess is that it was a requirement from Toyota. Not sure how they decided that made sense to them, but it certainly made sense for me ☺.