I'm not saying that this "que" is wrong (because I honestly don't know). But I know that it's uncommon and it sounds very weird. We can just ignore it or substitute this "que" for "se fosse":
A superfície do lago é como se fosse um espelho.
So am I. Did you report the error? Well, I did, but my impression is that wrong sound files hardly ever get fixed.
All the errors in Duolingo turn language learning into an unecessary struggle with inconsistencies.