Increased Duration Support And Ballista Support

After using a stopwatch to time how long my Ballista Totem connected with Toxic Rain lasts, it's clear Increased Duration Support does not effect the totem at all, even though the literal line of the support gem is "supports any skill with a duration".

Is this intended or a bug?

Edit; I would guess this is because the ballista totem support doesn't have the duration tag. Seems very confusing to word Increased Duration Support as effecting ANY skill with a duration, but not really mean that.