I know.
I'm saying the members worshipped Ymir.
It's very possible that it was one of the members considering what it said.
And if it was one of the members, then it makes sense why he would bow down to their "Mother"/"Goddess".
The question then is: How does it know?
There is no way we can speak of a coincidence in regards to that scene and the flashback of Grisha.
People thought Ymir's people were some tribe or another settlement.
But now we know Ymir's People were an Eldian Movement worshipping an ancient titan across the sea.
Either way:
Ilse strongly resembles Ymir, the girl always with Historia and is a shifter.
Yet, it's actions refer to that it was referring to the goddess the movement worshipped.
And even then, how high are the odds that it's mere chance both are called Ymir? I dare say zero.
That's why I claim that there must be a connection between Goddess Ymir and the Ymir we know.