Bill Zebub
“It’s probably Matt Mercer’s fault.”
Think about this, do you want "challenges" that are just scenes for your party to have guaranteed victory at no cost or risk? You can play this way and be guaranteed participation ribbons all campaign long.
I love fights where you feel like you're on the verge of a TPK the whole time. And I want DMs to both create encounters and run the monsters to make them challenging. But it's annoying when the challenge comes via questionable rulings.
For the scenario in question, I would have ruled that the fiend still gets two attacks because jumping is movement, but with disadvantage for the irregularity of the attack, cancelled by advantage for the prone target. And had the player announced their plan before executing it, I would have told them ahead of time how the ruling would go. (Which is an example of why it's a good idea for players to explain their reasoning before trying to be too clever with their action declarations.)
P.S. "participation ribbons" isn't a very respectful way to make your point. Just sayin'.