Except the claim was that there are more than one strain and the author is responding to that. The burden of proof lies in the one making the claim that there are multiple strains. The author has no burden of proof.

This would be akin to one person making the claim: "Unicorns are not only real, but are causing a global pandemic" and then someone writing an article titled "Unicorns aren't real, STFU". The second title doesn't have the burden of proof and we wouldn't be having this conversation if these were the titles of the respective articles.

Why is the unicorns title an accurate analogy? Because the science doesn't expect there to be multiple strains in the same way the science doesn't expect unicorns to exist. I'm using the same vernacular here that we use in science. I didn't say "unicorns don't exist" but "we don't expect unicorns to exist". This is how we say things.