Dwarven Henchmen - speed and encumbrance?

[Derek Vande Brake]

PRD wrote:

A character can lift as much as double his maximum load off the ground, but he or she can only stagger around with it. While overloaded in this way, the character loses any Dexterity bonus to AC and can move only 5 feet per round (as a full-round action).

PRD wrote:

Dwarves have a base speed of 20 feet, but their speed is never modified by armor or encumbrance.

Is the speed effect from being overloaded considered a special rule, or simply an encumbrance effect? That is, can a dwarf still move 20 feet despite carrying double his "maximum load"?

[Driver 325 yards]

The rules for lifting and dragging seem to be separate and apart from encumbrance, which is based on the type of armor you have on and the weight of your gear.

So, it seems like the dwarf cannot lift double his maximum load off of the ground and move at his speed.