For whatever it's worth, Memory Alpha does say
Corporal Askwith was in Starfleet, anyway. Of course, they also call him an officer, despite Corporal not being an officer's rank.
That's actually not worth very much in itself. Memory Alpha is a great resource, but it's ultimately simply a collection of user-generated content. It's full of mistakes just waiting to be spotted and corrected. There have been a number of occasions where I've quoted or had quoted to me a "fact" or date from MA, only to find when I actually checked the episode (or transcript, and I've found errors in
those, too) for myself that it was not in fact accurate to what was shown and said therein. People make mistakes, jump to conclusions. The various editions of the Okudas'
Chronology and
Encyclopedia (again, while wonderful to have for reference) were also peppered with errors and dubious assumptions. The official website has had mistakes as well. And yes, the episodes
themselves sometimes contain mistakes!
Askwith is never identified as a Starfleet officer in the dialogue of "The Forge." He is
assumed to be one on the basis of appearance. And that is not a strange or unreasonable assumption to make. But the "United Earth Diplomatic Corps" shoulder patches
combined with the non-Starfleet rank make me suspect there
could be more to the story than "United Earth embassies are guarded by Starfleet personnel." Which is why I would be interested to know what if any additional details there might be in the script to guide interpretation of what's seen in the episode.
Speaking purely hypothetically to illustrate a point, it's entirely possible that, as
@Mr. Laser Beam posits, Askwith and his fellows were described as MACOs in the script. Maybe someone raised the (possibly spurious) issue of the MACOs being confined to Earth before joining Archer, causing a perceived need to hastily change their affiliation, and the best the costume department could do at that point was slap UEDC patches on some SF uniforms. It's equally possible that they were described in the script as Starfleet, and someone raised the (possibly spurious) issue of SF being an agency of exploration rather than a military force, leading to the same effect. Or maybe they were non-descript "Embassy security personnel" in the script and the costume department had to make its own call as to how to handle them. Maybe a variation on one of these, or some other scenario. The Reeves-Stevenses or Mike Sussman might be able to tell us more about what went on behind the scenes.