Gentoo is considered stable

Honestly, I'm too unfamiliar with Gentoo to make a proper assessment on this. Though, even my (simple) understanding allows me to understand it as follows:

• Gentoo is not a point-release distro. Hence, by definition, it satisfies the definition of a rolling-release distro.
• Furthermore, regarding Portage's branches, this page suggests only two branches; Stable and Testing. With Stable being set as the default one. Furthermore, AFAIK, tests occur for over a month over at the Testing branch before updates enter the Stable branch. Hence, more time is taken compared to other rolling-release distros.

Which, I believe is what's alluded to here: "The update philosophy of a distro is generally not related to its release cadence, as you can have rolling release distros that are relatively stable (for example, Gentoo) and point release distros that are relatively bleeding edge (for example, Fedora)."

Is there any reason why you would deem Gentoo as not stable? If so, what?

but fedora “leaning unstable”?

For the sake of completeness, proper quotation would have been "leans bleeding"

I'll give you that the article is definitely not exhaustive and/or properly clarified. Perhaps for the sake of brevity, idk. Hence, I believe that this confusion is justified. However, again, I think the raised point is justifiable based on the following:

• Fedora is known to push new tech first. Heck, it even adopts it first; e.g. PulseAudio, systemd, GTK4, Wayland, PipeWire etc. Hence, this causes Fedora to feel bleeding edge; i.e. because its users are literally the first to test it en masse.

May I ask why you think Fedora does not lean towards unstable?

Anyway what is that whole un/stable supposed to mean anyway?

I agree it causes more confusion/conflation that it has any right to.

All non-rolling distros try to be stable.

It depends on the used definition of "stable" 😅.

• If "stable" is used here in the context of name for used branch (of the repository). Then yes, but this also satisfies rolling-release distros; as they, by default, ship software with their own designation of "Stable" (even Arch). (For the sake of completeness, I'm aware that some distros default to testing/unstable branches.) Hence, using this definition of "stable" is not very productive.
• If, instead, "stable" is used in the context of stability. Then, also yes. And, yet again, this also satisfies rolling-release distros. It's not like any reasonable distro is out there to deliver software that's known to cause issues and whatnot. The distros only differ in how exhaustive their testing is. Which gets us to...
• If, finally, "stable" is used in the context of how well-tested the distro is. This also ties in to the earlier presented definition for name of the used branch (of the repository). Because we all know that Arch's Stable repository is wildly different from Debian's Stable repository. And here, unsurprisingly, we find wild differences that are also actually helpful in a productive conversation.
• (Surprise,) tied to the previous point, "stable" could also refer to how often the distro requires you to update. With "stable" being used to indicate that updates are only required between (infrequent) point-releases. However, non-intrusive security updates should be able to get through regardless.

What can break are third party repos and stuff you compiled yourself.

Sorry, I can't agree with you on this. Even if this is said in the context of non-rolling distros, my experiences with Fedora suggest otherwise. Granted, Fedora is sometimes referred to as semi-rolling release distro. So, perhaps it (and direct derivatives) are the exception.

With fedora that can “break” twice a year.

Agreed (with earlier mentioned caveat*).

With a rolling distro that can “break” on every updates

Agreed.