not sure I understand the reasoning here because the behavioral response to covid at a collective level will always depend on the harmfulness of the disease regardless of when symptoms set in.

We are going to take stronger measures against harmful diseases, and that reduces their fitness inevitably. The claim that a disease always evolves towards being more harmless is obviously not true due to mere chance, but in the long run there is a clear advantage to it. Rather than cherry-picking examples to prove the point I think it's more accurate to say the article cherry-picks counterexamples like multiple Malaria strands competing against each other in the same host or a rabbit disease that became more deadly to overcome an immune response. None of which is long-term adaptive.