Calliope reveals here in no uncertain terms the true nature of her secret furthest ring hideout; a vortex from which no information can escape* is in fact a black hole. The signature shape of solar material spiraling into a black hole probably gives it away, but this is a pretty clear demonstration of the extent of Calliope’s ability to distort space before her more iconic use of the power in Act 7.
With this in mind, the stage in the middle of the vortex makes it quite a bit more obvious that Calliope’s Act 7 black hole is in fact Homestuck itself.
* Calliope may be dismayed to find that information paradoxically can leave a black hole, and that this is the same principal that leads to angels splitting into two.