> the researchers presumed that 30% of individuals who contract the new coronavirus never develop symptoms but remain 75% as infectious as those who do develop symptoms.
They made up some numbers and played with the math. It's conjecture, not research.
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.
They made up some numbers and played with the math. It's conjecture, not research.