Will Deore M6000 SGS derailleur work with 42t cassette in 1x10 configuration?
I talked to Shimano this morning and got the final word. The reason the long cage  is designated as having a shorter reach than the medium cage is because the long cages is designed to be used with the triple crankset.  The Shimano  representative  I spoke with confirmed that the long cage is indeed only designed to wrap around 36T cog.
The bottom line is the medium cage m6000 derailleur  is the only 10 speed derailleur designed to work with the 11-42t cassette out of the box.