Attack on Titan s3 e3

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Isn't the theory that Ymir can transform because she ate a Titan that could a bit odd seeing as she was human in the first place?

I've forgotten already if she told someone that story or we just saw her remembering it?

If no one knows that part of her story is it a wrong theory about Titans eating over Titans? Or did they just change the law of the world? Or is what ever punishment handed out to Ymir a one way ticket to being a Titan (and no one knows about it) and eating a transforming Titan was her way out of it (and no one knows how they got those powers in the first place!)
I'm just gonna say in the manga historia receives a letter by the end of this arc that reveals what Ymir's story was in season 2, though they still tried to hide some details. Ymir has already told riener and berdtholt that she was a titan for 60 years until she ate their friend, eren heard them before they realised he woke up. The titans becoming human after eating humans is canon, but it only works if that human is a titan shifter with special powers/features. A few things have been shifted about though, namely eren and historia being kidnapped quite soon this season and the death of the Reeves boss. This season, the mp already have E&H when the Reeves boss is killed but in the manga these happen at the same time, the Reeves boss agrees to use the pair as bait to draw out the government
So part of the mystery of what the Titans are has seemingly been resolved. They're humans that have been turned into Titans either by punishment like Ymir, or by whatever special skill the Beast Titan has (as seen by Connys village), or by having the power to switch.

Reiner and Berdtholt were talking of going home. Do we know if that is the same place Ymir was from and thus the source of all the Titans? Erwin was talking about the lost history of the humans, maybe that would have revealed the truth.

Or what ever is behind the door that Eren has the key to!
what's behind the door, which is likely where this 24 large series will end up, explains everything. a lot of stuff makes sense. a few things make sense by about halfway through as well, but revealing what's behind the door could be compressed into an episode I think, the other one halfway through could be too but that will leave a bit more time for something else if it's compressed in the same way. sticking with this season will pay of, not just for the reveals either