Marc says he wasn't clued in on producers' true intentions to expose the network's toxic workplace claims ... explaining on the "Elvis Duran and the Morning Show" that initially, he thought this was just an innocent look-back on his Nick days.

However, he claims the producers pulled a bait-and-switch maneuver on him ... allegedly hitting him with some footage that they asked him to react to, which was unsettling.

MS says ... "They ambushed me. They never told me what this documentary was really about. And so they showed me a video of something that I couldn't believe was on Nickelodeon. And I said, 'Well, let's stop the tape right here. What are we doing?'"

When they finally spilled the beans that the doc would be diving into actor Drake Bell's teenage sexual assault by Nick dialogue coach Brian Peck -- along with accusations against Dan Schneider -- Marc says he threw in the towel and walked out on it completely.

6 weeks later, Marc says he was told he was nixed from the doc. But then, 2 weeks after that, he says he was told he was in the first part of it 'cause he spoke positively about Nick.

In his portion of the doc, Marc makes clear that all the allegations being levied happened after his time on the network, when he hosted "Double Dare" from 1986-1993.