Hi,
Just tested it today on Beta. sephuz 's proclamation DOES stack with orc racial: hardiness but it DOESNT stack with relentless trinket.
with lego + orc racial, 6 sec kidney lasted 4.3 sec (29% reduction).
with lego + relentless trinket 8 sec sap lasted 6.4 sec (only 20% reduction)
TLDR the tooltips are actually right and stuff stacks in the way you would assume from those tooltips. Which is actually suprising in the case of Blizzard.