with the Dark Brotherhood?
Could Bethesda not remove that dialogue when you became leader, or did they just want to make her seem like she's lying about having any ties with them?

No, she definitely has ties with them. So I would say it is an oversight.

She has ties with them, but it is possible she has not gotten word that you are now their leader, she still threatens you if you are leader of the thieve's guild *facepalms*

Because she thinks she's such hot sh... substance.

Well, there is a way around that. If you told her you're the leader of the DB, she would threaten to send that information to Imperial authorities and then it would be destroyed once again. With the Thieves Guild it is even worse, as they need her protection.

I say fuq Maven Black-Briar, i hate the Bit**

Really wish we could slice her troat in a killcam mode haha