The reason why these are mostly believed to be false positive is that all current covid-19 lineages (sequences) can be traced back to a single clone that (based on mutation rate estimates) appeared somewhere in late 2019. There may have been other lineages before that, but we seem to have no RNA sequence of those lineages, so we cannot put them in relation to any existing strain. One has to wonder, where are those 'alternative' lineages now, if they existed in the first place? Sure, they may have gone extinct in the great lockdown. But then one has to ask why did those went extinct while so many other virusses didn't. An a simple (occams razor) explaination is: they never existed.

It's a very safe assumption.

We can look at excess deaths (https://www.cdc.gov/nchs/nvss/vsrr/covid19/excess_deaths.htm) and the speed and locations it spread around the world and be fairly certain that if the virus were here in significant quantities in 2019 we'd have seen signs of it.