The second definition from Wiktionary is most appropriate for this situation:
> (formal) A supposition based upon incomplete evidence; a hypothesis.
There is no evidence that asymptomatic people are 75% as infectious as symptomatic people. The 75% number was fabricated to allow them to build a model. Asymptomatic people could be 0% as infectious as symptomatic people. Or, by some unexpected turn of fate, they could be 200% as infectious as symptomatic people. The evidence doesn't exist.
Your link even mentions another scientist who believes that asymptomatic people are only 35% as infectious.
Do you know what conjecture means?