Note it is possible to loose muscle before losing fat thus eating just junk food will have a negative impact on all parts of your body because you won't get the needed macro nutrients.
No. We didn't evolve for millions of years with a stupid system that catabolizes muscle before fat. You will always (unless you use vitamin S) lose some lean mass when losing fat. Most ratios are 4:1 (4 pounds of fat to 1 pound of lean mass) or 5:1. Whether you lose weight fast or slow, that ratio tends to hold true. Anecdotally, it held true for me whether I did it the proper "Body Building Way" (Slow at .5 lb per week) or super fast (3 pounds per week). In both cases after a prolonged diet (84 days), my overall body composition was the same after a refeed and waiting about 1 week.
I think people pretty much forget what fat is and what it is for. It is "storage" plain and simple and your body is NOT stupid. It isn't going to say "Hey, lets eat up all this lean mass and "SAVE" this huge reservoir of fat". No, fat is a fuel that is constantly at work.
Additionally, it is pretty damn hard to lose lean mass. You can certainly lose it, but it is difficult. Stop lifting, starve yourself to 5% body fat, stay there for a few weeks extra, don't touch a weight again, and then shoot right back up to 20% BF. Yeah, you will be a different person. The Minnesota starvation study showed that true muscle loss (more than 4:1) did not occur in these men UNTIL they essentially depleted their fat reserves (entered true starvation, estimates were in the range of 5% before they starting losing lean mass at a much greater rate).
Whether you are an evolutionist or a creationist, believing the body will burn muscle before fat is pretty stupid, I think if you rational approach the topic without all the misinformation and hysteria.
Also measuring muscle loss can be extremely inaccurate, depending on glycogen levels. the only way to truly know (which I have done with self study) is to diet two different ways (clean foods, vs junk foods) and then refueling back up and then take measurements. This gives the body time to restore homeostasis and weight and will give you a true idea of what you actually lost (using calipers). Even though, though, this isn't precise, but is much better than when people test immediately after a keto diet and notice a huge loss is muscle (6-10 pounds of water will be gained back after returning to a normal diet, mainly of glycogen and retained water). This is when you measure, not when depleted, as that misreports muscle loss. It takes a pretty solid refeed if you been low carbing or even just a low caloric intake for a while to return to what I would call "normal" glycogen levels and I don't believe most published studies take this into account.
Junk food is bad because it has additives and man-made stuff, is caloric dense, and is crafted to make you want to overeat it! So, it will most definitely be harder to diet with it... But it is possible. Read the twinkie diet experiment by a guy - should be somewhere on the net.