We're still developing some of the aspects of the Border Router functionality so we may not be able to satiate all your curiosity.
So have you guys ported the spinel protocol for using it in ESP32?
We're not using the Spinel protocol at the moment. Part of the reason is because we want to have a consistent network stack between the Argon and Boron. The Boron runs a binary and is not user-programmable. Therefore, we couldn't include the other half of the Spinel protocol needed on the Cellular radio. We're using a more standard link-layer interface. If all goes according to plan, we'll stick with it, no need for Spinel.
OpenThread needs a border router to communicate with the external networks such as WiFi...Is ESP32 in the Argon board doing all the border router work?
Correct about the role of a border router in Thread. The nRF52840 on both the Argon and Boron provide the Border Router functionality. They're simply using their Wi-Fi/Cellular Network Coprocessors for the outbound link.
That could explain the decrease in the number of OpenThread devices under a usual network from 250+ to 50+ in Argon board.
Maximum network size is based on a several key concepts and is more than just this device or that. See the reply here for details.
HTH!