It depends on who the attackers are. If both characters have 50% armor, they will mitigate that much damage without taking armor penetration into account. Different attackers can have different armor penetration, so the final mitigation isn't always the same. If it's a special damage attack, you'd go off of resistance instead.
But in terms of Bossk specifically, he has +50% defense while taunting from his unique, and his lead gives bounty hunters another 100% while at full health. 50% armor is about 638 flat. So another 150% is 1595. This is about 71.4% mitigation before armor penetration is taken into account.
So no, Bossk and Nebit are not the same.
