I bought one of this file from Home Depot:
https://www.homedepot.com/p/Nicholson-6-in-Slim-Taper-File-21866N/100188490 And sanded the three long corners to 'safe' them. I use it only for crowning. It took a bit of practice but it works on any size fret, and you can see what you're doing while you're doing it.
I can see how the rods with sandpaper over them would grind against the side of a fret if you slid it the same direction as the fret wire sits on the board, but you can't see what you're taking off, you have no control over the shape of the fret as you're rounding it, and you would definitely sand into the fretboard. I would say this kit is not worth the trouble. There are some awesome (and expensive) crowning files out there, but there's nothing wrong with going old school and just using a three corner file. Might I add, fretwork is not a job in which going faster is necessarily better.
EDIT: I am watching the YouTube video... this guy has quite a cowboy approach to leveling and crowning frets, and his video offers no real proof that the frets are anything other than shiny. I'm not saying it
can't work, I just don't think he's getting the results he claims he's getting, nor do I think that 99% of the people who buy the product will get the results he claims he's getting.
Also, what
@telemnemonics said: when crowning, you absolutely don't want to touch the tops of the frets. If you knock material off the fret tops, you're going to be out of level, period.