The Executive Summary [0] cites the 1990 maritime boundary agreement between the U.S. and USSR [1] to justify its placement of the western ECS border. In that agreement, Article 1 says:
> Each Party shall respect the maritime boundary as limiting the extent of its coastal state jurisdiction otherwise permitted by international law for any purpose.
And Article 5 clarifies:
> For the purposes of this Agreement, "coastal state jurisdiction" refers to the sovereignty, sovereign rights, or any other form of jurisdiction with respect to the waters or seabed and subsoil that may be exercised by a coastal state in accordance with the international law of the sea.
So in the U.S.'s view, the agreement clearly extends to ECS jurisdiction over the seabed and subsoil. The remaining question is whether Russia will continue to repsect the agreement that was made with the USSR.
[0] https://www.state.gov/wp-content/uploads/2023/12/ECS_Executi...
[1] https://www.state.gov/wp-content/uploads/2020/02/US_Russia_1...