> can even be formed from (and treated similarly to) lambdas

They are also used to create lambdas (even the shorthand stabby-lambda syntax desugars to a call to Kernel#lambda with a block.)

> Ruby does separately have callcc for more precise control over stack unwinding, although it’s a little known feature.

callcc is included in CRuby but has been sidelined from Ruby as a language separate from CRuby as an implementation for a while, with Fibers understood to cover the most important use cases for callcc.