https://www.deviantart.com/adorkastock/art/2021-DEJ-20-Shh-872504691 Is an image where you might be tempted to do an S curve. An S curve isn't wrong.
https://www.dropbox.com/s/cpmwrlwczzkev81/IMG_0042.JPG?dl=0 Is two different takes (by me) from that same reference. So yes, you can use an S curve. But for me, it's harder to continue with the sketch I started with the S curve. It's tougher to judge whether I got the curves right, and it's hard to add on straight lines where they should go.
The goal with the tutorial is not to give hard and fast rules. It's to point you to areas where it's worth exploring. So focusing on simple curves and straights, maybe trying a sketch that's all straights even if it feels like an S curve would be easier... that will teach you things. Or on an image where it feels like it's full of straight body shapes, try doing it with all simple curves, see where that gets you.
There's no objective wrong answers in gesture drawings. But there's answers that are wrong for you.