@jtl What “method”? Let’s Encrypt just sends the leaf certificate plus the X3 intermediate signed by ISRG Root X1. The second path you’re seeing on SSL Labs is figured out by SSL Labs itself: the leaf certificate mentions http://cert.int-x3.letsencrypt.org/ for potential download to complete the chain. While this isn’t necessary (the X3 signed by ISRG Root X1 is send by the server already), it does enable clients to complete the second chain.
I’m sure this is something that happened “by accident”, as all leaf certificates have the URL to the X3 intermediate signed by DST Root X3 embedded into them.