Yes, that summary is correct, modulo the language "rooted at".
The ACME API will offer two chains for clients to select between when downloading their newly-issued certificate:
- leaf <- R3 <- ISRG Root X1 <- DST Root CA X3 (self-signed)
- leaf <- R3 <- ISRG Root X1 (self-signed)
The first of those will be the default, which clients will get unless they have been configured to request an alternate chain.
The second of those will be the one used by the ACME API to conduct TLS handshakes when clients first connect to it, since we do not have clients which run on Android and so do not need the extra compatibility provided by the default chain.