Go, the language Caddy is written in, does not know anything other than elliptic-curve Diffie-Hellman. Neither something like ssl_dhparam can be set, nor are »1024 or 2048 bits« used.
! go/cipher_suites.go at release-branch.go1.9 · golang/go · GitHub
In other words, there’s plain RSA suites without any perfect-forward–secrecy (PFS) and ECDHE_{RSA,ECDSA}. Nothing else. Not implemented is any EDH_{RSA,ECDSA}.
If anything like eq 3072 bits is displayed, then the SSL check site likely means ECDHE with a 256b curve has been used.