Uh.... Poison isn't the same as acid. Author, the poison would've just coated the rope in poison. If this was an acid vat, then at least make it consistent and say he had acid resistance....
And while I get it that he got hit with his fight or flight and instinctively went for flight. I'm surprised he didn't take the opportunity at that time to try to kill it once he discovered it feared the "poison"-with-acid-like-property...
I also question how the skill protected his clothes from being affected by the acid-like-poison but not the rope. Very selective skill... Really the way this should've been properly resolved is that he should've remained bound, he should've realized that he had nowhere to run, he should've done a desperate move and barely managed to make the creature swallow poison. Gained a ton of levels from it, and then either had the brute power gained from that to free himself or go back to the dead guard and use their swords to cut the rope.
I don't mind reading slop, but at least make slop that follows through properly. This feels like we're already taking on a side quest to finish the tutorial when we could've just finished the tutorial.
And also, just because he is protected from the effect of poison (and apparently Acid like effects of a poison). This doesn't explain how he isn't falling over melted and dead once he went out of the acid-like-poison pool, because he's still drenched in poison. He doesn't miraculously become dry.
This is some herculean amount of ass pull right off the bat.