It came from this
entry in the Comics Should be Good blog. Looking back at the quote from Alan Davis:
"The irony is that the
Multiverse was created out of Marvel UK’s desire to use Marvel US characters–which wouldn’t have been tolerated at the time. Setting the stories on an alternate Earth offered the opportunity to use any of the Marvel stars without concern for continuity–but Dave had no love for Marvel characters and Alan was offered SWAMP THING before he had finished his first arc so we never took advantage of the possibility."
It's not clear at what time that was occurring so maybe the comics you're talking about came after. Alan Davis was working with the previous writer to Moore, Dave Thorpe.