That is what I thought too, but it is literally the only mention of a dragon 'escaping' that anyone could extrapolate anything from.

On another note:

Quote Originally Posted by Nalien View Post
If there's anything resembling a plot hole, I'd say it's the idea that he was protecting a seal on Primals at Silvertear, when he catastrophically failed to do that. Again though, I don't think that's what he was protecting, and I'm more incline to think releasing Primals was an intentional act. He wanted to stop Garlemalds advance, and releasing Primals did exactly that... At least it did for a while...
While I do believe 'protecting the seal on Primals at Silvertear' was the ORIGINAL intention for Midgardsormr, it seems clear to me that they may have changed that idea when they decided primals as we know them were not actually real gods but rather ideas given thought, life and form through belief and aether. As for puposefully realeasing the primals to stop Garlemald... no I personally don't think that flies since my impression of him is that of loyalty to Hydaelyn. Rather Cilia's explanation of Middy being a bit too full of himself and making a big 'woops' seems more likely. Especially since his tone towards us when we first met is that of arrogant condescension.