I think there's a few things at play here. One of them is the tier list. It exists for a reason, and the community consensus on roughly where the characters are isn't for no reason. Of course that doesn't mean that by picking up Fox you automatically win every tournament, but it does give you extra options if you're able to become good enough with the character. What Armada has been able to do with Peach is use her to the absolute maximum and show what she is capable of, but that doesn't mean her weaknesses go away. In the current era where you see Mango, Hbox, Armada and M2K in losers before grand finals, it's clear that the gap is closing. But it's notable for me that Armada has been seen at recent tournaments struggling with Foxes (aside from Leffen/Mango) in a way that was just impossible a few years ago. (By struggle I mean the matches are close, or even dropping games, instead of the Fox destruction we've become accustomed to) That isn't to take anything away from Fox players like Hax who are doing an incredible job, it's just that we've come to expect that unless you're in the top 5, your Fox doesn't have a chance in hell against Armada. This isn't the case any more.
This leads to another point. Armada has been prominent for so long (apart from the hiatus) that anyone wishing to counter his style literally has hours and hours of videos of him playing to study. Everyone approaches the game differently, but there are very clever people around. Given enough time, it's not unsurprising that eventually people have started finding small cracks in Armada's usually impenetrable defence. Westballz's tactic in their recent set shows this. There was virtually nothing that Armada could do about camping the top platform and it was only the character differences that allowed this to happen. As soon as Westballz tried anything else, Armada got the upper hand. and eventually won the set. Matchups aren't static and the metagame changes over time. Another example was Hbox trying Ness, Falco and Fox before eventually beating Armada's Young Link with the original reason for YL, his Puff. It took several tournaments (and about 3 years?) before Hbox was able to best Armada's YL, but is it a surprise it eventually happened? Not to me it isn't as Young Link is by far the worse character. He had particular traits that allowed him to deal with Puff, but only because Hbox hadn't figured out how to beat the strategy.
To me this is something that seems to be happening to Armada's Peach now too. It's not like it's a crisis and he's losing to everyone with it, but it was clear from Apex that his Fox, which he can't possibly have put anywhere near as much time into as his Peach, was able to get him games and sets against the other top 5/6 that I don't think his Peach would have been able to. In the winners semis set, Armada only got one of the games by doing an incredible coast to coast string of hits on a PPMD mistake. Although Armada eventually lost in GF Set 2, the Fox was able to pressure the Marth a lot more. Armada said on stream after grand finals that he feels that his Peach has reached as far as it can go. I would be highly surprised to see Armada take another major tournament with Peach.
The thing is, you can know exactly what a Fox is going to do in a lot of situations, but because the character has options it's harder to predict and you can use that to your advantage. Peach, being slower among other things, isn't able to mix it up quite as well. The mixups come from the mindgames and the players rather than the character. Amsa is a really good example of this too. He plays a character way down on the tier list and has done amazing things, but MLG Anaheim showed how people were able to exploit Yoshi's disadvantages in a best of 5. Yoshi is a worse character and the tricks that Amsa does are incredible, but the MLG sets showed that numerous players were able to figure out how to beat a "worse" character in the space of only 5 games. As I said, people have had years to counter Armada's Peach.
To make it work, there has to be a noticeable skill difference. In Armada's dominant era, you had Mango not quite at his best, PPMD still improving and not quite at his peak, Leffen was nowhere, M2K was inconsistent and Hbox had no idea how to counter Young Link. But now, with the skill gap among the top 5/6 being so close, the character disadvantage becomes a bigger factor. It's 20XX in action for sure, though other characters are obviously still viable, but it does seem as though Armada's Peach has gone as far as it can now.
Though people have said Marth is a counter to Fox. Leffen doesn't like the matchup and PPMD did win most of his matches with Marth.
