I don't think this was a robust case at all. A couple of things to note here:
1. The witness from Iceland whose testimony against Assange has now revealed to have been lies wasn't part of the original case the US put forward. They added him much later into the proceedings. So if this was a "robust case" as you put it, why would they decide to add new accusations?
2. You might say, well, they didn't need to include him and shouldn't have. But you have the judge (Baraitser) citing these new accusations when siding with the US, as Stundin reported: "Baraitser sided with the arguments of the American legal team, including citing the specific samples from Iceland which are now seriously called into question." So clearly they were significant enough for the judge.
3. Craig Murray has written about why he thinks the witness was included late into the proceedings:
https://www.craigmurray.org.uk/archives/2021/06/fbi-fabricat...
> The hearings on the Assange extradition in January 2020 did not seem to be going well for the US government. The arguments that political extradition is specifically banned by the UK/US extradition treaty, and that the publisher was not responsible for Chelsea Manning's whistleblowing on war crimes, appeared to be strong. The US Justice Department had decided that it therefore needed a new tack and to discover some "crimes" by Assange that seemed less noble than the Manning revelations.
> To achieve this, the FBI turned to an informant in Iceland, Sigi Thordarson, who was willing to testify that Assange had been involved with him in, inter alia, hacking private banking information and tracking Icelandic police vehicles. This was of course much easier to portray as crime, as opposed to journalism, so the second superseding indictment was produced based on Thordarson's story, which was elaborated with Thordarson by an FBI team.
> The difficulty was that Thordarson was hardly a reliable witness. He had already been convicted in Iceland for stealing approximately $50,000 from Wikileaks and with impersonating Julian Assange online, not to mention the inconvenient fact he is a registered sex offender for online activities with under-age boys. The FBI team was in fact expelled from Iceland by the Icelandic government, who viewed what the FBI was doing with Thordarson as wholly illegitimate.
> [...] While Baraitser's eventual decision barred extradition on the grounds of Assange's health and US inhumane prison conditions, the second superseding indictment and Thordarson's accusations were accepted as a valid basis for extradition.
Given all that, I think it's fair to describe him as a key witness.