I think that’s correct. Individual LAN subnets will be /64 (derived from the /56 delegated prefix).
Here’s a snip of my config that successfully pulls a /56 prefix from Charter for the WAN (eth0) and gives a /64 to the LAN (eth1). Should work with Comcast Xfinity, too.
set interfaces ethernet eth0 address ‘dhcpv6’
set interfaces ethernet eth0 description ‘WAN’
set interfaces ethernet eth0 dhcpv6-options no-release
set interfaces ethernet eth0 dhcpv6-options pd 0 interface eth1 address ‘1’
set interfaces ethernet eth0 dhcpv6-options pd 0 interface eth1 sla-id ‘1’
set interfaces ethernet eth0 dhcpv6-options pd 0 length ‘56’
set interfaces ethernet eth0 dhcpv6-options rapid-commit
set interfaces ethernet eth0 ipv6 address autoconf
set interfaces ethernet eth1 description ‘LAN’
set interfaces ethernet eth1 ipv6 address autoconf