I would not allow that. It does not say, "Speed. Your base walking speed is 25 feet. Your speed is not reduced due to the weight of heavy armor."
"Heavy Armor. Heavier armor interferes with the wearer's ability to move quickly, stealthily, and freely. If the Armor table shows "Str 13" or "Str 15" in the Strength column for an armor type, the armor reduces the wearer's speed by 10 feet unless the wearer has a Strength score equal to or higher than the listed score."
Dwarves just aren't affected by the bold portion there. If a, 14 strength dwarf that was unencumbered put on plate, he would not slow down. If he was 1 pound under encumbrance and put it on, he would not slow down due to wearing heavy armor without the appropriate strength, but he would slow down due to encumbrance. A non-dwarf in the same circumstance would drop 10 feet for not being strong enough for the armor and 10 more feet for the encumbrance.