The Swiss actually had a long history of buying foreign weapons and licenses to build foreign weapons, they had for example artillery from Krupp, St. Chamond, Bofors, Bohler etc. in their weapons inventory at various time periods, Gatling and Maxim machine guns, Luger pistols etc.
I have seen that video of the AAC antitank gun being fired, it is shot from the rear and it unfortunately doesn't really show the features of the gun from the side or front. I am trying to find out more about it since it supposedly had a US Army M number, M21 I think, assigned to it, which would indicate that it had been standardized, but apparently it never went into full production, which also happened to some other weapons in WWII that had M numbers assigned too, such as the .45 Hyde M2 sub machine gun for example.
The gun in the Swiss photo is a single gun on a mount that resembles the old 37mm Puteaux trench gun mounting, but it is larger in size.