Large social networks already have API's with partners, but who and how they are allowed to be used is defined through legal agreements and access controls. The access act is intended to regulate how those legal agreements over the API should be defined.
The EFF wants to allow third-party to ask consent of those users in order to use those API's for interoperability. In theory this would not put any additional requirement on the social network, and the consent request could occur out-of-band. The only change would be the legal ramification and access control.
In theory there could be a dominating social network without any API for third-parties, in which this proposal might be defeated. In practice that is very unlikely given the need of large companies. The proposal could also require if needed that internal API, if they exist (which all of them have as a matter of practicality), get opened up in the lack of any external API that can be used to enable interoperability. I would however guess that such a broad brush would be problematic, and a bit unnecessary since again every large social network already has third-party API's.
Do Discord have an API that a selected group of partners has access to? Do Instagram have it too? Given the correct permission, interactions between those two networks would be a matter of a third-party using those API's.