More information...
I found more information regarding the Enduro Max
6901 and
7901 bearings at
www.bikeparts.com.
One difference is that the installation of the 7901 appears to be directional with an inner (black seal) and outer side (blue seal). The Surly docs say the black side of the 7901 goes out.
Playing the part of junior mechanical engineer I am going to guess the 7901 directional install may have something to do with different radial vs. axial load handling on each side of the bearing.
It appears the 6901 would work in the short term. I guess the risk is lower than expected service life.