Asserting that there is absolutely no temperature dependence without evidence is pretty much the same thing as asserting there absolutely is without evidence. If there is no data, the best choice is to say that you don't know while coming up with plausible hypotheses. That a virus might share some traits with other closely related viruses is not so uncommon. Extrapolation from prior evidence when no direct evidence is available is one of the main capabilities of the scientific method. This is pretty far from a 'made up story.'

It's a bit of moot point though. Because of the extremely plausible nature of the idea that covid might have some temperature dependence, there are many people investigating this specific claim. With a cursory web search, you can find several papers and pre-prints on the subject, a small majority of which find evidence to support that trend.

Saying that temperature can't affect covid transmission because Singapore is having a tough time is akin to suggesting climate change isn't real because it snowed in April. It is just one data point.