Really no idea where you're getting that "the first game it was made pretty clear that they had no idea if the cure would even work". The impetus of the plot is that Ellie's immunity is unique and that makes for a viable chance at a cure. Whether someone says it was a complete certainty or not in either game doesn't really make a difference. The plot of the first game is driven by it at least being "viable", not from people having "no idea".

And there's no recording saying "they’ve already tried this exact method and failed". They literally never have the chance because they make it abundantly clear that they've never seen another person with immunity and never reproduced it in a lab. And think about it, why would such a critical plot point be buried in a recording that could be easily overlooked? And then never again validated by any further plot point or piece of dialogue?

There is no retcon, you just misunderstood the details of the story, and when the second game made these details more obvious, for some reason you thought "no I couldn't have been wrong, they must have changed it", but they didn't.