There is no distinction between spells and "debuffs". What's happening here is just that Sunder Armor triggers a different spell. In this case, supposedly due to the Weakened Armor effect being "normalised" these days and triggered by several abilities.
http://www.wowhead.com/spell=7386
http://www.wowhead.com/spell=113746
Also, GetSpellInfo only works with names if they are in your spellbook. If you use spell ID it will always work.