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That's not a failure, it was just never finished.

And more because of personal matters than anything technical. I'm not sure why this is relevant.

Of course it was relevant. If they had a better game engine it would have been further along.

The article you yourself linked says the game's development troubles were from pressure to surpass their previous game which was a smash hit.

How would a better engine fix that?

They didn't need "a better game engine" to create Celeste.