Eh.
His run speed isn't amazing. His aerialspeed/accel are just average. His options out of walk are broken but his walk accel is lol.
Options out of run aren't that great. Has a lot of trouble catching people with flips(Flip Jump, Bouncing Fish, Monkey Flip) because of how slow he is, despite having a long disjointed sword. His OoS game isn't great. His frame data is only average(but I'd put it at above average because swordie). His dash to shield is godawful. His jumpsquat is just average. He has to approach almost everyone and play on most of the top tier's terms.
I don't disagree that his neutral is good, but it's not as spectacular as you're making it seem.
There's not much here that makes his neutral look better than any character out of
.
When you're playing Marth, there's no question of "how the hell am I going to hit this guy without getting hit myself," which is what a good neutral is all about. Top 5 neutral seems like a huge stretch to me, but if you changed that to top 10-15 it wouldn't seem as questionable.
His options out of a run are stupendous.
The horrendous dash to shield length is the design shortfall to counteract the significant amount of options he has. But like many design binaries, usually a high/top tier still comes out on top due to the strength of the positives
(shoutouts to bayonetta).
Diddy Kong's options out of a run are a lot worse. He only has one stand out option: rising fair (or rar bair) with ZILCH aerial acceleration and with a banana some shenanigans (that people aren't letting Diddy get away with at top level as much anymore), rolls and dash grab are good but just like him jumping can be, primed to be baited/read and punished.
Why does ZeRo struggle against certain people/characters? Because Diddy's mid range footsies are kinda average - if you can circumvent forward air (Ally is godly at this, Marth kinda does this a lot easier than most of the cast) Diddy has to take more risk than his kit can make up for vs many higher characters - his oos options are basically limited to who gets hit with fair by being upright in their end lag. How can you circumvent fair? Well, it's like -20 if he has to land with it, and it can struggle to hit people on the rise, it's made to be near indomitable in it's line of fire but is still not an all encompassing tool (one of the best moves in the game, don't get me wrong - it's what allows him to reduce so many characters options in so many situations; like ledge guarding).
Marth has rising aerials, landing aerials, one of the best SHADs in the game, and can act before touching the ground on two aerials - so jumping out of a dash; he has [close to] the best available choices overall. Mewtwo may have a stronger claim, Sheik doesn't have a SHAD game but is probably the overall most dominant in the SH paradigm.
Dancing Blade has shown itself to be one of the best moves in the game; again. As it can be done out of a run and is
transcended priority, he has something that will beat just about every poke or anti-approach even from those with comparable or better range - unless they also have a sword. Anyone notice how consistently Leo's (Mr.E does it too) dancing blade beats/punishes diddy fair? lol.
A largely disjointed pivot grab, the lovely pivot ftilt, long length rolls (average speed though), a very good full hop game (meta definitely kinda spurs in this direction - Diddy doesn't have a full hop game - or at least his fair becomes near useless within that sphere, and Megaman takes more risk applying pressure vs full hop tactics, Fox beats grounded/SH games with full hops but can't challenge others also at FH height, etc etc): how many characters can hit people on the ground on the rise with a full hop? I honestly don't know for sure but it's likely very uncommon.
And then of course there's to consider that although his dash2shield is slow, that his oos options are pretty potent, tilts/side-b/aerials, up-b, etc.
This isn't like Pikachu who has a below average dash to shield but with little to no solid dash options whatsoever.
His run itself isn't very fast, but he can still mostly chase - he's beyond that tipping point of being slower than certain characters in the air. Roy's niche is to be faster than Marth and sometimes you can feel that power of what having a faster running 'marth' can be like, yaaa it feels good LOL; but I think it's safe to say that his run speed is adequate nevertheless.
With things like back air and up air having amazing swinging angles/arcs (hitting horizontally far above his head) - he punishes flips with less risk than most characters. I feel like you might be generally misconstruing that flips aren't inherently difficult to punish. But with fantastic priority, comes great opportunities (Rosa/Luma's up airrrrrrrrr).