Usually user feedback drives priorities. For platform is it development teams as user. Or really product (DocuSign) end user? How does this all connected?

Yes, customer feedback is one of the most critical inputs to prioritization. For Platforms, you typically focus on 2 sets of customers: 

1. Direct customers: These are users of the platform. Exact customer persona will depend on the specifics of the platform. For e.g., if you are building an ML platform, your typical persona would be data scientists, ML engineers.

2. Indirect customers: These are users who will see the value of the work your direct customers are doing. 

Depending on whether you are building an internal or external facing platform, level of focus on these 2 segments of customers will vary.

As Platform Product Manager, you focus on direct customers using your platform but also focus on the business impact it has on indirect customers. Hence, it is recommended that you have a good line of sight on business impact as part of your platform roadmap.