> I don't think this is a big reason Gentoo users use Gentoo.
Exactly. The two most compelling reasons to use Gentoo are/were: 1) It's a rolling-release distribution and 2) it offers customization by virtue of building from source. I don't think I've seen the argument that it's more secure--not seriously anyway. I used it back then because I wasn't a huge fan of Linux and Gentoo seemed more familiar to me with its ports analog.
Perhaps part of the OP's confusion is the hardened profile (or similar)? I'm not sure considering their wiki currently advertises it as risk mitigation [1], but I haven't used Gentoo in probably 6-7 years (at least not consistently outside a VM) so my memory on this is likely wrong.
[1] https://wiki.gentoo.org/wiki/Hardened_Gentoo