All Eco Mode does on the 3700X is reduce boost clocks by 100mhz and cut power consumption by 10w. You also need to have PBO enabled in BIOS to use Eco Mode.
https://www.techarp.com/computer/ryzen-eco-mode-guide/
https://www.techquila.co.in/testing-echo-mode-on-the-amd-ryzen-7-3700x-80-less-power-for-99-performance/