no it should be there, that means that it is also not compiled, there might be something wrong with the config, may be there are some dependencies for this config to be enabled.
I did a trick in the PKGBUILD to compile custom configurations. Create a dir called custom in the directory where PKGBUILD is. and copy arch/arm64/configs/rock-5b.config from src/kernel (afair) with the name config.
So you can edit this config however you want and PKGBUILD will use the config in ./custom/config.
There is a whole lot guide in Gentoo Wiki on how to configure zram module in the kernel that may help. I might not have access to device this week, but i think this info might help to troubleshoot.