You are correct, and I am too: the OFFICIAL nameplate on the building, as well as their OFFICIAL email address show: "Ambassade - Mission permanente auprès des Nations Unies" (Royal Embassy of Cambodia - Permanent Mission of Cambodia to the United Nations)
Chemin Taverney 3, 1218 Grand-Saconnex, Geneva, Switzerland
(41) 22 788 77 73
camemb.gva@mfaic.gov.kh
https://pmcgeneva.mfaic.gov.kh/about.
Also note that . 1) the French wording is on their plate at Grand-Saconnex; 2) French is the language spoken at Grand-Saconnex and in the Canton of GENEVA; and French is one of the two Official working languages of the UN to which the Cambodia is accredited. Best regards