Do you really think so? The study author is quoted saying when they started 'it was clear almost overnight this was not manmade'. As for the 'probable', 'likely' wording etc that's just typical scientific writing, due to the epistemological underpinnings of science. We never actually prove anything, so arguably it isn't correct to use the verb to be without a modifier (e.g. 'likely') when describing study findings.
There wouldn't be a way to definitively show this wasn't man made, its one of those prove a negative situations. What they are saying is that many of the parts being used are unknown, which would make it really unlikely.
Its like if you got a weird vegetable in China and wanted to figure out if it is GMO or a new natural species. If there are genes very similar to known ones from jellyfish, and bacteria, it is likely that it was a GMO. However if the new genes in it are vegetable origin but not related to known species, it is really unlikely it is GMO, but not impossible:
It would require the Chinese scientists to have 1) found new animal/plant species the rest of science didn't know of that were not closely related to anything else, 2) have kept them a secret before they knew there was anything useful in them, and 3) Put them together in ways that just aren't used in genetic modification science yet.
Going back to virus, scientists in China and elsewhere work full time to find new novel Coronaviruses + other potentially zoonotic disease causing virurses and sequence their genes, and they've published on thousands. Very unlikely the components in SARS-CoV-2 would be novel to us if it was manmade unless they've been keeping a massive research effort secret from the world somehow. It's giving mad scientists way too much credit, we know how advanced the gain-of-function research was in China as just a couple years ago the leader of work in the field was in the US.
