Does it imply that the Dark Powers are more powerful than Takhisis?
If so, then why didn't they nab Takhisis herself?
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The Dark Powers are indeed more powerful than certain deities, at least the demigods. For example, they were able to fetch Vecna, then a demigod, into Ravenloft. More details about the dark powers can be found here.
Deities are powerful on their own realms in the planes, or in the crystal spheres with the majority of their followers. So while the Dark Powers are not likely to be able to capture a powerful deity like Takhisis from her own realm, if she happened to enter the mists, she would arguably have a hard time leaving it without outside help.
I think the second point also implies that once a non-deity enters the demi-plane, even deities cannot pull him/her out without the approval of the Dark Powers, as the demi-plane of dread is the realm of the Dark Powers, where their power is supreme.
In short, we can perhaps liken the power-level of the Dark Powers to that of the single-sphere overpowers like Ao of the FR, or the High God of DL. (Read more in the 2e Planescape accessory On Hallowed Ground.) In its own crystal sphere, an overpower's rule is absolute: Ao is known to invite gods to FR, to imprison them in avatar form to the prime material, to organize them, to kick them out. But outside of its chosen sphere, an overpower is almost a noone.