Please provide proof of this actually happening (whether it is a news clipping or youtube vid or whatever) or shut the fuck up.
Maybe you should shut the fuck up, since you haven't offered any evidence to prove that I'm wrong aside from that Guinness never verified something, which doesn't actually mean anything. You can read about the volume of their shows on Wikipedia, among other places, and you can also find show reviews, articles on things like the volume of their music making people vomit and resulting in things such as police involvement to shut down shows by them due to the volume, live videos, live albums and various other things online, so I don't feel pressured to provide evidence to support something that there's way more to suggest is true than is not.
Besides, considering the fact that Guinness discontinued their listing and support that record after 1984 out of fears for ear damage that could result from bands trying to be the loudest ever, which is the period of time that Swans played their loudest shows, that means absolutely nothing. So you're basically telling me that because they weren't recognized as holding a record that ceased to exist before they would have been eligible to receive it, they can't possibly be louder than Manowar.
Besides, The Who held that record with 126 decibels in the 1970's, and since many bands, including Swans, had gear and amps that dwarf anything that existed back then by a good margin, it's logical to assume that they could be much, much louder, and since Manowar apparently wasn't much louder than The Who to begin with, I don't see how it would be unreasonable that a band that played an already loud kind of music with extremely large amp set-ups wouldn't be louder than them.
Just go to YouTube and look up live videos by them from the 1980's. They're definitely louder than you seem to think possible, because they caused people to feel physical pain and vomit, and people would call the police to report disturbances pretty much any time they ever played live.
They got a lot of attention from the British press because of how people reacted to hearing their music live, something you can look up to verify on your own. I know there's even video interviews on YouTube where Michael Gira talks about it, and you can probably find some original articles if you look for them.
For additional info, try actually looking up the band.
I have a feeling that there are other bands who were similarly loud, such as early SPK, but none of them gained more than a cult following during the 80's while they still played music like this, so Guinness probably didn't even realize they existed.
Nowadays, I'd wager that something like Sunn O))) or some Boris shows are definitely extremely loud.
EDITING FOR SOME YOUTUBE VIDEOS AND OTHER STUFF
Very loud and very slow live videos:
Compression definitely lessens the impact, but you should get the idea. In the 1980's, the volume they played at was typically in the range of 130-140 decibels and sometimes even higher, depending on the setup of the venue they played in. It's only just recently that Manowar played a show at 139 decibels, 2008 in fact, while Swans played at that volume and higher in the mid-1980's, after that record was no longer supported by Guinness.