Good question, I don't remember this specific circumstance coming up before.
I'm trying to think of things to compare it to, the only thing I can think of right now is if you play KTL(Kill The Leader), or ABL(Anyone But Last)it's ok to attack a worm that isn't leader or the last team as long as you also attack the leader or other team during the same turn. Although that's not really a league thing.
I think it would be ok to do what you done since you started your attack while on the rope, it's actually clever what you done, as by the rule you did attack from rope since your attack started from being attached with the rope.
The main reason i'd be ok with this is i've seen many other situations over the years where you begin your attack from the rope but your worm lands before the weapon actually hits.
You are allowed to knock worms, which is attacking from rope, yet not attached while actually hitting the worm.
You can throw a grenade then hide before it lands and hits, or, you can throw a grenade then alter it's course by throwing yourself into it.
These are allowed so I see no reason to object to what you done, pretty sure I could come up with a lot more examples if it wasn't 7:15am and i'm getting ready for bed lol.