That bit you quoted is apparently a typo, is should say -10.
A +15 Chain Hauberk (AT 16) could reduce the minimum penalty from -25 to -10 (and the max from -130 to -115).
(Note: A hauberk is a very long chain shirt, a type of armor, not a weapon.)
There is also an a rule that says that says Strength may be used to reduce these penalties below their normal minimums (page 142). However, there are actually 2 sets of Encumbrance rules in RMC Character Law.
One set can be found in the Special Modifications section of Table 10-04 on page 137, and it basically gives a a single, smaller adjustment that is not modified by any skill or the rules found on page 142. In these rules, armor is counted as part of the encumbrance.
There is the second, more detailed set of Encumbrance rules on page 142, and which is combined with the separate Armor maneuver penalties, and the rule regarding Strength reducing the modifiers below even the normal minimums.