This sums up the way I think about it, but it gets at what is really worth noting here. Match-ups are not about tournament results. They are about the options each character has, and what of those options are completely shut down, or amplified in efficiency by, the other character's options or lack thereof.
Young Link has projectile options at pretty much every angle between straight up and straight down. Combining this with his mobility, he really has one of the strongest pure zoning games in Melee, and it limits a lot of Jiggs' zoning options. Jiggs can't just space a full jump bair against Young Link, because he has to avoid the boomerang, and if he manages that, he can't punish standing too close by ff bairing because of the bomb. This is a set-up Armada used a lot, and Hbox didn't have enough speed up and down to punish Armada for running away when Hbox managed to close the gap.
Now, it's possible that Jiggs has options to beat Y. Link's zone, but Hbox didn't display any, and Armada is (understandably) demurring. As far as the match-up goes, Y. Link's options appear to negate a lot of Jiggs' options. Ergo, the match-up is pretty good for Y. Link. I don't know enough about either character to say for sure, but all I've seen anyone offer is empty naysaying rather than real options that Jiggs has against Y. Link.
It's also worth noting that based on understanding of the game, match-ups can and do change. Early on in the lifespan of BB:CS1, it was thought Hazama had a bad match-up with Hakumen, because the latter could cancel out any of the former's pokes and turn them into a large hitbox to frustrate approaches. But then, as Hazama players discovered new mobility options for Hazama and the openings in Hakumen's pressure became better understood, the match-up was seen as much closer to even. So there's no reason that the match-up can't be a soft counter until some option for Jiggs is figured out.
