I saw that press clipping as well and I suspect it might be a bit misleading. It's my impression is that the 'chart positions' they're citing are just for the stereo versions titles that have quad counterparts, because the three titles they list are positions #1, #2 and then #6, so presumably positions 3-5 didn't see quad release. If it were some kind of dedicated quad chart, surely they would just list the #1, #2 and #3 sellers?
I've been buying quad stuff on ebay since about 1998, and the Isley Brothers have been on my radar for the majority of that time and I've never seen a copy of either the SQ or Q8 of this title, and I've seen multiple copies of all the other Isley Bros. quads in that time, including being outbid on at least 3 copies of the Q8 of Go For Your Guns, a title that was the dying wheeze of CBS's quad program in 1977 and probably only in print for a year or so at most.
In an interview in Billboard circa 1975, Al Lawrence (who was head of Columbia Quadraphonic A&R by that point) said that all their quad releases had a 'built-in audience' of about 25,000 and that sales between vinyl and 8-track were split about 60/40. That means for even the lowest sellers from the CBS associated labels catalog, there should have been at least 15,000 LPs and 10,000 8-Tracks created. Surely if 'The Heat Is On' was a #2 top seller, at least one quad copy in either format would have surfaced by now. Even noted quad collector
Nick Perugini has The Heat Is On listed in both his matrix vinyl and Q8 want lists.
A quad mix of The Heat is On may very well exist, and maybe it even made it to the point of being pressed somewhere, but to me the evidence says that this album never saw any kind of full-scale quad release. It's my hope that there's a pristine discrete 4 channel master of this album sitting the Sony vaults (along with the other Isley Bros. quads) just waiting for digital release, because as FredBlue won't shut up about, er, I mean has said before on here, it's amongst the very best of the albums that The Isley Brothers ever did.