They customized it some, but it's all more or less upstream condoned code that Jason built.
Also, if you want to access your tailscale network, but don't have permissions to create a tun or wg device, the fully userspace implementation can work in that situation, which seems like a nice property to have.
Also, Wireguard is really easy to implement, making it less of a problem to have multiple implementations. Each implementation is more likely to be correct/invulnerable.
Small implementation was a design objective of Wireguard, after the horrors of IPsec (see Linus' email that praises the difference).
Also, it's based on code by the wireguard author: https://git.zx2c4.com/wireguard-go
They customized it some, but it's all more or less upstream condoned code that Jason built.
Also, if you want to access your tailscale network, but don't have permissions to create a tun or wg device, the fully userspace implementation can work in that situation, which seems like a nice property to have.