A lot of it is JWest playing his character to the fullest, which if I recall him saying, it was something like "Get a hit, reset to neutral over and over" and then his opponent makes a mistake and he kills them. (I'm assuming by "hit" he means anything besides Dash Attack at low %s. Ha)
He is famously known for eating every single Luigi ever known to man. I have no idea if Samus is favorable in that matchup but he makes it look that way. He placed pretty well considering he went all Samus at Genesis (I think it was 65th? I'm not sure I want to yell "MU inexperience" at this point considering this was like, the hardest Smash 4 major ever for a lot of reasons).
A large part of me believes Samus is misunderstood as a character and no, I'm not saying she's not bad, I just think there are a ton of misconceptions about the character and a lot of them don't hold true, unfortunately. One of her most redeeming factors is that when she does actually get a hit from neutral and it's a good hit, she usually converts for ridiculous damage (40% off of a burst movement dash attack is one of her MODERATE NORMAL punishes, for perspective. You can even sourspout the dash attack and you still get guaranteed combos, the only prerequisite is that you space it properly and play Samus and not Falcon.)
I equate Samus to a lovely rollercoaster. It's awful waiting to go up to the top but you get that hit and it is amazing because her advantage is ridiculous compared to the rest of her character, all things considered.
I personally find her a lot more fun than Zelda because in spite of all this, when I get a hit I'm pretty firmly rewarded with the usual guaranteed damage and good positioning that comes with Samus getting a hit. Beautiful~.