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The set comes with 4 colorations: brown/gray with yellow glow from Ulduar 10 man, and dark gray/gray with red glow from Ulduar 25 man. These 2 sets are complete.
The third and fourth color sets are DK's T8. T8 sets don't have boots as part of the tier. WotLK didn't have transmogrification so I guess they didn't think to "complete" the DK's sets with matching boots.
Wowhead's 3 screenshots, 1 with blue skin, rest two with the original skin.
The color was changed, I assume after WotLK.
The screenshot on Wowhead showing the boots is from destroyed SW, which happens in Cataclysm. Also the screenshot's url explains it as well:
https://wow.zamimg.com/uploads/scree...-25man-set.jpg
"boots of the underdweller the newest color skin matching the dk t8 25 man set"