From reading that, it lets you establish connections within your tailscale vpn. Iroh let's you establish connections between devices regardless of their network.
I think everyone in this thread agrees on that part already.
The similarities are in an application lib to connect, and that tail net IPs correspond to device keys like in Iroh. The service using the Go library has its own Tailscale identity.
There might be a misunderstanding of what Tailscale offers here. There is no "VPN" in the classic "virtual network" way. With Tailscale, you can - as with Iroh, IIUC - connect arbitrary nodes to each other, where a node can be a device or an application (via tsnet). All nodes get CGNAT IPs and an addressable hostname, so there is one giant "network" of all your nodes with automatic DNS resolution baked in.