I just had an ARAM game where the enemy inhibitor was respawning and the enemy Bard ulted his 2 nexus turrets to save them. While they were in the stasis of his ult, the inhibitor respawned. Even after the inhibitor was up, (and our team was pinging the inhibitor to signal to destroy it first), we could still destroy the nexus turrets while the inhibitor was still alive.
Im pretty sure you can hit the nexus turrets if there is no inhib turret anymore, even if the inhibitor itself its still up. Though its fairly uncommon, since they are well armored and its hard to tank them since they do quite some damage.
usually you destroy them AFTER the inhib because minions walk near turrets and they automatically reduce their armor, so they are easier to kill.
Maybe the case in your game was that while the inhib was down, some minions passed the inhib and went for the turrets, reducing their armor. After the inhib respawned, the turrets were still easy to kill due to the leftover minions. Then after you cleared turrets, you went back to get the inhib to expose the nexus.
What you cant do is to actually kill the nexus with the inhib up.
