I really can't see it as even when I just look at their tools. She has about 3 or 4 reliable moves. That's it. She is better at spacing and has better ledge trap options, but Marth has better pressure, better punishers, better frame traps, better throw set-ups, he is a much better killer and heavier. Overall I would say he has the better movelist. Running speed is equal IIRC.
She has some nice combos which is great for damage racking, but nothing else she has is substantial enough to make it even IMO.
She can save her side b for killing, but then what is she going to space with? Bair? Marth's moves have equal range to that or more depending on the attack. And overall she has less safe moves then him. She has Bair and side b. That's it.
No, I feel comfortable giving Marth the heads up on this.
EDIT - Sorry for typing so much; I love thinking about matchups because it makes me think about my character critically and gives me new ideas.
"3 or 4 reliable moves" is an unfair assessment imo. ZSS has lots of very good moves, but just b/c they are good doesn't mean you can lead into Marth with them. She has to lead into him with her spacing game and try to only allow him through it when she can punish him with the rest of her moves.
Just to talk moveset for a sec: her dash attack, all her tilts, her u/f/b-airs, her jab, neutral B, side B, up B, d-smash, and u-smash are all useful moves, but not in all situations or matchups - they each have a time and place though, so knowing when to use them is the key and why it doesn't look like the best moveset on the whole; the blessing of using these moves is it removes decay on her side-B. If you space for a bit, then brawl with her other moves for just long enough, you can bring your side-B back up. And b/c her edgeguard game is so strong, you don't always have to straight-out KO with the side-B anyway: getting them off the stage to a proper edgeguard is often good enough (chasing with a side-B off the stage is also very low risk and an easy kill if it lands; if it doesn't, it can often mess with recovery plans and lead to a gimp).
I totally agree that Marth has a better overall game than ZSS. He's way more well-rounded, no question. But if you look at all the "games" you listed there, the one that's imposed first is ZSS's spacing game. Marth can't pressure, punish, or throw set-up before he addresses the spacing. I know the spacing can be predictable, but it is sort of a mindgame of its own: you have to keep worrying about it b/c if you don't, you are going to get clipped. And it when you get a little too used to being spaced that ZSS stops spacing and utilizes the rest of her underrated moveset (her grab can be a good way to get this going - it's easy to forget about it b/c it's rarely used and it may not combo into anything, but it positions nicely). As kind of a side note, I'd say ZSS has better mindgames (or maybe she is more reliant on them?).
As far as killing goes, when I play this matchup (and I have played it probably close to 100 times), there are times when it's hard to kill Marth, but there are also times when he has trouble killing me; the difference in weight and KO power (which I think you underestimate on ZSS) is offset by ZSS's ability to play very strong defense. She should be able to stay out of tippered fsmashes early - yes, they happen, but so do 40-60% gimps of Marth b/c of armor pieces (that shouldn't happen either but stuff that shouldn't happen does once in awhile) - and the higher her % goes, the more she can fall back and try to exploit aggression, which is something she does well. Marth can pressure her very well, but it has to be calculated pressure or you will get grabbed or dodged/PS'd into a tilt.
To close: I'm fine with 55:45 Marth just because he's a little less hot and cold than ZSS, but I just can't see this as an "advantage" match. I feel like the better player takes this, not the better moveset (the person better versed in the matchup could steal this though - a Marth who's never met a good ZSS might be taken aback, while a ZSS who is used to being underestimated might be surprised at a Marth who knows her tricks).
And I understand that you (Emblem) are the more experienced smasher in this discussion, so I'm willing to yield out of respect for my betters (but I don't think I'm a scrub... I'm able to keep up with a local guy who has placed 2nd to 5th in regional tournies a number of times anyway) Pardon any poor assumptions I make about Marth. My brother (who plays all this Marth that I play against) and I are very equal in skill and we are always very close in this matchup, so that might be why I feel it is so close. When I say I know Marth well, it's more like I know his Marth well.