I had previously bought two of the same “not-Girardis” from the same nursery as @Shibumi and ended up developing the same suspicions because of the large distance between internodes. In order to compare, I bought two more Girardis from Lucille Whitman at Whitman Farms (a lady who definitely knows her mulberries). The internode distance on the WF Girardis was noticeably shorter.
I posted pictures of the two types here for comparison: Why Gerardi is a dwarf mulberry - #107 by marten
As @Shibumi mentioned, the consensus from the more experienced mulberry growers was that only the WF Girardis were genuine.