Warmongs Passive and Soraka W: Buggy Coding or Special Exemption?

I noticed a game ago that when i built knights vow on soraka its passive would disable warmongs passive because you "take damage". why is this so when Sorakas own W, her heal, dosnt disable warmongs? im assuming that abilities that use health dont count as "taking damage" aginst warmongs passive but if so than warmongs should state in its description this rule and the rule that items like knights vow will disable it.

some clarification from Riots end would be much appreciated.