`apt-get source` from own `sources.list`

The Server Fault answer 447536 involves on Ubuntu 18.04 (more precisely on avocat.ovh.hw.ipol.im):

apt-get update -c some-dir/etc/apt.conf

Hit:1 http://ppa.launchpad.net/certbot/certbot/ubuntu bionic InRelease
Hit:2 http://ppa.launchpad.net/ubuntugis/ubuntugis-unstable/ubuntu bionic InRelease
Reading package lists... Done

apt-get source tree -c some-dir/etc/apt.conf

Reading package lists... Done
E: You must put some 'source' URIs in your sources.list

apt-get install tree -c some-dir/etc/apt.conf

Reading package lists... Done
Building dependency tree... Done

No apt package "tree", but there is a snap with that name.
Try "snap install tree"

E: Unable to locate package tree

grep '^deb-src' some-dir/etc/apt.sources.list

deb-src http://archive.ubuntu.com/ubuntu/ bionic main restricted
deb-src http://archive.ubuntu.com/ubuntu/ bionic-updates main restricted
deb-src http://archive.ubuntu.com/ubuntu/ bionic universe
deb-src http://archive.ubuntu.com/ubuntu/ bionic-updates universe
deb-src http://archive.ubuntu.com/ubuntu/ bionic multiverse
deb-src http://archive.ubuntu.com/ubuntu/ bionic-updates multiverse
deb-src http://archive.ubuntu.com/ubuntu/ bionic-backports main restricted universe multiverse
deb-src http://security.ubuntu.com/ubuntu/ bionic-security main restricted
deb-src http://security.ubuntu.com/ubuntu/ bionic-security universe
deb-src http://security.ubuntu.com/ubuntu/ bionic-security multiverse