Just tested this, and it does actually prevent Exhaustion and Fatigue without issue so long as they are sourced from enemies.
I suspect this might be (semi) WAI.
WAI because there are various effects that are "cast" by the players themselves, or proc'd by the players themselves and these need to trigger (such as the Barbarian Exhaustion after Raging, as if I remember right, only the high level Barbarian ability Tireless Rage removes the Exhaustion effect on dismissing it or after it lapses). There are various other existing such effects that bypass similar immunities and thus I suspect it is likely WAI because of that (Bracers of the Demon Consort giving neg levels bypassing any immunity other than Racial: Undead).
It "might" not be WAI as well, since we don't know if the Devs intend to permit this to preven the Rage exhaustion/fatigued effect after Rage finishes. However, given the above points, I suspect it would be more likely to be the former (WAI).
J1NG
