I contacted them about an issue with one of the other National Motorcycle Museum listings, an Indian Westerner, because they had it listed as a twin cylinder when it's a single. They promptly corrected it and said they have someone working through the listings trying to find any more issues. There's certainly a disconnect between people putting up the listings and people familiar with the motorcycles, but Mecum handles such a wide variety of bikes I imagine some level of detail would be hard to get past, you can't have an expert for everything but you can certainly count how many cylinders are on an engine.
Here are the details of that Moto Guzzi, as provided from the collection itself:
https://nationalmcmuseum.org/2023/03/10/1928-moto-guzzi/One thing that has always driven me nuts at that museum, the Moto Guzzi logo upside down on the fender of this bike:
https://www.mecum.com/lots/1088612/1948-moto-guzzi-gtv/I don't know if that's correct or not, but it looks wrong to me.