As a Gentoo user for the past ~quarter-century, I'd say that it's more that -unlike Debian- Gentoo has been using a system service manager that's way better than the classic SysV init for approximately forever.
The early discussion of systemd-as-init [0] was pretty much 100% focused on how much better systemd-as-init was than classic SysV init. When restricted like this, systemd-as-init is an obvious winner. But, when you consider other init systems -such as OpenRC- that provide a bunch of useful scaffolding and support tools (rather than demanding you reimplement all that yourself) the benefits of using systemd-as-init are far less clear.
I've mentioned this before in an HN comment or two from way back when, but I'm really mad at myself for not recognizing how extremely important the "What should Debian adopt to replace the incredibly ancient SysV init?" discussion was and failing to take part in it. OpenRC was knocked out of contention for reasons that were never really clear to me, and I'd have loved to put a bunch of time and effort into fixing whatever deficiencies the Debian folks believed made it unworthy of consideration.
Oh well.
[0] ...as well as some-to-much contemporary discussion...