US intelligence never examined the physical server, the cyber security firm was a client of the DNC, intel community simply accepted the findings without verification.
Much better Dutch article here[2]. It even mentions Muller.
[1] https://arstechnica.com/information-technology/2018/01/dutch...
[2] https://www.volkskrant.nl/wetenschap/dutch-agencies-provide-...
https://www.thedailybeast.com/exclusive-lone-dnc-hacker-gucc...
seems like intel community did plenty of their own investigations into this.