If spot instance price is the only communication channel between you and the cloud provider to achieve this, it's hard to do a good job at it. For example, if the spot price was $0.60 one hour ago, and $0.55 right now, and you still have 13 hours of latency left for your job's execution, should you start triggering it or not? (how do you figure out your bid level?) You could have statistical data to have an intuition what's the lowest price that's been hit historically on similar days of the week in the past etc, but it's inexact and overly complicated.

If the cloud provider becomes aware of the remaining latency you have at your disposal for the job's execution, they can do a much better job. They would be able to look across the entire job execution queue in that datacenter, each job having a specific remaining duration for its execution, they would know the predicted pattern of carbon/solar/wind split for the next hours, and the implementation of the system would sit just on the cloud provider side (making the life of the customers easier and simple).

Of course, in the end, the benefit is towards our planet, but this is much more likely to succeed if the proper cloud API exists and the implementation initiatives are properly aligned to avoid redundant implementations on the customer side.