> those weren't conspiracy theories but alternative explanations of the known facts supported by at least as much evidence (not much, not conclusive, but neither had the alternative) as their explanation.

This just isn't true. There wasn't anywhere near as much evidence for the lab leak theory as their was for a natural origin which is why the lab leak theory was determined to be unlikely compared to other theories. Even the people who were pushing the conspiracy theory admitted to the lack of evidence.

Tom Cotton, one of the first in politics to push the theory, said himself: "Now, we don’t have evidence that this disease originated there, but because of China’s duplicity and dishonesty from the beginning, we need to at least ask the question to see what the evidence says" (https://www.cnn.com/2020/02/18/politics/coronavirus-cotton-f...)

For example there is an early article from a 'low credibility' right biased news website (https://mediabiasfactcheck.com/daily-mail/) The Daily Mail, and even they had a microbiologist explain: "at this point there's no reason to harbour suspicions' that the facility had anything to do with the outbreak, besides being responsible for the crucial genome sequencing that lets doctors diagnose it" (https://www.dailymail.co.uk/health/article-7922379/Chinas-la...)

Same story with this article which was pushing the bio-weapon theory. (https://www.washingtontimes.com/news/2020/jan/26/coronavirus...). It also stated that evidence was incredibly weak without dismissing it entirely: "In principle, outward virus infiltration might take place either as leakage or as an indoor unnoticed infection of a person that normally went out of the concerned facility. This could have been the case with the Wuhan Institute of Virology, but so far there isn’t evidence or indication for such incident."

Meanwhile, other theories being talked about at the time were supported by actual science such as genome analysis (https://www.nature.com/articles/s41591-020-0820-9) and note that here too they weren't totally dismissing any other theories either, only presenting the best evidence we had saying:

"Although the evidence shows that SARS-CoV-2 is not a purposefully manipulated virus, it is currently impossible to prove or disprove the other theories of its origin described here...More scientific data could swing the balance of evidence to favor one hypothesis over another."

> The most prominent event was probably in September 2021, when Lancet published a rebuttal to their February 2020 article flatly declaring any non-nautural-origin theories to be "conspiracy theories".

The opinion piece February did not flatly declare every non-natural origin theory to be a conspiracy. What it said was "We stand together to strongly condemn conspiracy theories suggesting that COVID-19 does not have a natural origin." which is very different from saying that all theories of a non-natural origin are by definition conspiracy theories. Those same authors made that explicit in a followup article (July 21) saying: "Careful and transparent collection of scientific information is essential to understand how the virus has spread and to develop strategies to mitigate the ongoing impact of COVID-19, whether it occurred wholly within nature or might somehow have reached the community via an alternative route, and prevent future pandemics." They did not dismiss that other options were possible. They were just saying "We want evidence, not conjecture"

> Can you show me an article in NYT or WaPo in the period between February 2020 and September 2021 that describes natural origin and lab leak theories as equal possibilities, each of them could yet be proven true and each of them being legitimate alternatives and areas of scientific enquiry?

They weren't equally supported by the evidence, but there are certainly cases where articles on those (left-biased) sites made it clear that a lab leak was possible and that the origin of the virus was not certain.

https://www.nytimes.com/2021/05/27/health/wuhan-coronavirus-...

https://www.nytimes.com/2021/06/14/science/covid-lab-leak-fa...

https://www.washingtonpost.com/opinions/2021/05/27/medias-de...

https://www.washingtonpost.com/politics/2021/05/30/scientist...