How to cheat Set into believing the Supermutants are destroyed in the Watershed?

Looks like no one knows that it is possible to complete the Set's quest of destroying the supermutants at the watershed by just telling him so and cheating him to believe it is true (with high enough Speech). Since not a single walkthrough mentions this, instead all of them call for obedient watershed supermutant extermination as the only way to complete the quest. However this another undiscovered option does exist. The only thing - it is not always available through the dialogue with Set, that's why it seems no one mentions it. I completed this option and successfully received the Garrett's rewards from the fridge without actually killing a single supermutant at the watershed. The only question here's - I don't know what does really trigger this one? I remember I was running through the locations in the linear order (Shady Sands - Vault 15 - Raiders - Junktown - Hub - Necropolis) - and this one was available. In another game when I changed the order, this option disappeared from the dialogue and the only left solution remained was to kill the watershed bastards like all the walkthroughs advise. So I am really curious what is the real trigger for this option of this quest to appear. Does anybody have any knowledge on this?

Are you sure this wasn't from a modded version?

Absolutely sure. The regular version 1.1 I never played any modded version - just 1.0 and 1.1

It kinda sounds like a bug actually. You can report success and get quest rewards without actually doing anything? Might not be related to speech at all.
Maybe it's an error only present in 1.0, and/or hard to reproduce consistently?

I got this scenario the first time I played after a long break so I took it for granted. My character design wasn't the optimal (I forgot I could get 2 LK increase from Chuck instead of 1 and 2 IN from Lorrie instead of 1 so I maxed these up to 9 instead of 8) so I restarted. To my amusement, I couldn't reproduce the mentioned "peaceful" resolution of the Set's quest which was irritating. But then again I did many things differently this time. I pretty much recall everything I did the first time, and the sequence, too. I doubt it was 1.0 because I think it's hard to go through Junktown completing all the quests without getting bugged with extra characters and Junktown comes before Necropolis and there were no bugs at all in that game so I think it still was 1.1, but still not 100% positive, maybe just 90%. Unfortunately, since there are only 10 save slots in F1 I used that saved game slot for saving another game since I have taken this scenario for granted, as I said. The key difference between the 1st game when this peaceful quest resolution was available and the later games, apart from different character design, was that I had significantly more experience in Necropolis and significantly more time left before the water chip retrieval deadline the 1st time than later. And yes, exactly, I received the rewards without actually doing anything and having killed anyone, just having returned to Set a little later on and cheated him. There was 100% sure speech check as there were lines when he didn't believe me, saying something like: Is it done? - Yes. -No, I know it is not... So then I reloaded, raised the Speech a bit, entered the dialogue with him once more, confirmed the deed was done, he believed this time and ordered Garret to release the reward. I think it wasn't not a bug since there were many lines from him and from me to go through this scenario and it never looked bugged just very smooth and logical. I think I could replicate this scenario once more if to spend time on several attempts but the time is a bit of an issue right now so I looked into walkthroughs first amazingly without finding anything on this so then started this topic here