From the linked pdf in the footnotes, looks like they are rolling their own network protocol:
"At launchtime, the VPN protocol will be a Google proprietary protocol; however, to ensure a high throughput while minimizing battery consumption, we will soon adopt IPsec as the data tunnel protocol due to its native support in Android.
We may eventually use other protocols, such as Wireguard, as their native support improves or on platforms where no specific protocols have a specific advantage."
"At launchtime, the VPN protocol will be a Google proprietary protocol; however, to ensure a high throughput while minimizing battery consumption, we will soon adopt IPsec as the data tunnel protocol due to its native support in Android.
We may eventually use other protocols, such as Wireguard, as their native support improves or on platforms where no specific protocols have a specific advantage."