As far as I understand. Juggling is not a combo my friend. It's the act of keeping your opponent in the air. Your opponent can have control of themselves, but if you can keep them in the air w/o ever touching the ground. The entire time from starting to finish of you keeping them in the air with moves. It was juggling.
For Example: In melee, Marth can keep peach off the ground pretty well. But nearly the entire time marth is doing this. Peach has complete control over her direction and mostly she can fall fast or pop open her umbrella and fall slow. However if marth is still consecutive on keeping her off the ground even though it's not a true combo. It's still juggling.
So IMO Fire Juggling exists. Even if you can't do it with 100% Din's fire. It's definantly a move to help you keep your opponent in the air, and off the ground. A Combination of up-air, din's fire, and perhaps other aerials or ground moves can assist in juggling. and what is upair and din's fire made out of? Fire.
Hence Fire Juggling.
Like Marth uses F-air and Up-air to keep peach off the ground and have more control. Upair counts as a juggle move and so does f-air, even though Peach can avoid it.
Juggling = keeping opponent off the ground.
You sounded so sure of yourself about how it's Not real, and doesn't exist. Well Be creative, and learn how to juggle your opponent. If he airdodges? fine, find a way to keep him in the air. Din's fire is far from useless making it sound like it's as bad as luigi's fireball. "It's easy to dodge." You got more tools in your arsenal than to just stand there like a drone and spam Din's fire onry. if it keeps failing, use other tools to keep them up. and juggled.
Using only 1 move of anything is easy to dodge, if you use it over and over. Stay unpredictable, and as long as you succeeded in keeping them in the air. and if din's fire was one of ur tools regardless if it was dodged. It was a juggle.
Not to mention if it still works like Melee.
Where you can't airdodge if your out of jumps and tumbling. Then Din's fire can still hit regardless of the opponents airdodge cuz it can't happen. So therefore the juggling can commence. Now if it sets them free out of tumbling animation after 1 din's fire? well as long as you get more chances to keep them in the air it was juggling.
Again we aren't just spamming Din's fire here. The opponent has to take account for your other options. Otherwise I could apply what you said about fox's shine in melee. "Oh his shine is easily avoidable just save your jump and f-air him when he trys to jump out and shine you, there really is no such thing as shining someone."