Those numbers look about right to me as well considering how often we (don't) Backstab in Legion, to keep it competitive with MoS. If you want to get an idea of how much it would add to your DPS, take how much damage you do from Backstab in a log and multiply it by 1.635 (Gloomblade does about 63.5% more damage than Backstab against a target with 35% armor and no spell resistance... if a mob in PvE has spell resistance then you need to adjust for that too... I don't PvE anymore so I couldn't say). So even with 10% bonus from MoS, your Gloomblade would always do more damage than Backstab, regardless of positioning. MoS makes up the difference though by increasing ALL YOUR OTHER damage also.
And yes, that means that the guide is very badly wrong, and people will almost assuredly be confused throughout the xpac.
As a side note, I always worry that this kind of mass confusion that's just objectively wrong tends to drown out and cause legitimate criticism to be likewise ignored.