A DMCA takedown can only be sent by the owners or someone acting on their behalf. (Though, microsoft is more than fine on making exceptions for the RIAA in the case of youtube-dl I doubt they will make an exception of individual devs)
Legal twist: Since the project is under AGPL, this technically makes anybody who downloads/uses the project a copyright owner (you have the right to make copies), thus anybody could send a valid DMCA request to the repo.
Obviously we're a bit in uncharted territory and there's some nuances and things vary by jurisdiction, it should be receivable one way or another, and it would take 10 years in court to really determine it.
That is absolutely not how the AGPL nor (US) copyright law works. If I license someone else to make copies, they do not become a "copyright owner". You are allowed to make copies of the versions of the work released under AGPL, but not newer versions not released under those terms.
>>> ... but not newer versions not released under those terms.
That's exactly where shit gets really complicated ^^
1) The software couldn't be relicensed in the first place because there are parts for which they don't own the copyright. Can't relicense something you don't worn. The attempt to change the license of the software is void at best.
2) The AGPL is contagious, forcing all future contributions to be AGPL as well. Their claim that contributions started being under another license at some point is void, because these contributions were necessarily under AGPL.
Notice there's an interesting conundrum in how the two elements interact and create a deadlock. It's on purpose, the GPL/AGPL is meant to prevent relicensing.
It's copyright violations all over the place. There's gotta be a thousand and one ways to make them accountable. You could personally start redistributing/reselling the software assuming it's AGPL so you get some grounds and go after them for saying it's not.