So their explanation is this. The goblins in the MM are one example of a goblin. Goblins in another source (Phandelver psionic goblins) are another. They are two different creature types (fey, aberration) but still goblins. The goblins that are PCs (and by extension, NPCs) are a different type that don't have as strong an influence from the feywild as the MM goblins, thus are humanoids with fey ancestry (same as elves).
WotC has done this before: the eladrin monsters are fey, the eladrin PCs are humanoids. Why? The former live in the feywild and the latter lived on the Material plane.
Interestingly, this also is a workaround for people who don't like the lack of humanoid goblins. They can exist. They just use NPC stats rather than dedicated monster statblocks. Want a humanoid goblin? Grab the scout and make it a goblin. It also means that you can have gnolls that aren't fiendish and have become more civilized (and become humanoids) if you want.
The trick is to stop thinking of the stat block as the only expression of a creature idea.