I first saw this claim in YouTube videos by Dr. Jason Fung. A normal search for “loose skin jason fung” gives a bunch of hits (video and text) that are informative on the topic. I generally find him very reliable.
I believe I saw a short reference to loose skin and fasting in one of Jason Fung’s books. I hope we’ll get some other posters with more knowledge of this subject. I have concerns about loose skin, too.
IF conserves muscle better than CICO due to hormone response to fasting, so at least in theory, less volume reduction and sag, compared to CICO.
I would brace for some loose skin though. I heard people see loose skin shrink quite a bit over a 1-2 year span though. And you could also train to put on some muscle to further enhance the beach look.