The installer is actually borrowed from fedora, is actually a good installer with a few tweaks to make it work for sabayon a gentoo based distro.
If you were to install fedora, you would see the same installer.
I personally do not use the default installation, When I get to partitioning, I go down the list and choose custom partition.
I never use the text installer, since fedora does not have a text install, they dropped support for the text installer.
My point is, you will not see a custom (manual partition) option in the text install mode.
This imho, is why there were two spins created, spinbase with no x, forced to use a text installer and get a lvm for your efforts.
corecd was renamed to corecdx, you get a minimal x with fluxbox, and a working gui installer with the option to manual partition.
anaconda installer is a good installer, is good enough for fedora, It lacks a little for sabayon purposes.
In your case, I want to call it user error, But only for not taking the time to learn and understand what you were using
Hopefully this post will explain a little. You can do everything you want if you choose manual partition, on a gui install.
You will not get the same option on a text install.
I have not even looked at a default or text install, since fedora switched to a lvm.