They were celebrating when they did that. / They celebrated after they did that. / They celebrated as soon as they had done that.

[Marika33]

This woman said, "When they did this, when they did this surprise attack, when they took these hostages, when they murdered these families, they were celebrating".

Does she mean that "they" were celebrating when they did all of that? (After all, this is what was said!)
But that doesn't make much sense to me ... as I think they celebrated after (or maybe as soon as) they did (had done) all of that. But she used the past continuous and "when" for some reason . Is that a grammatical mistake of her?

I thought that with consecutive actions you should use the past simple. Even English Grammar in Use, by Raymond Murphy says this.

 

[jutfrank]

We can't know for sure exactly what she means but I suppose she means they were celebrating at that same time.