This type of lineart leans more into manga-style. I think it has it's roots in traditional japanese calligraphy. It ultimately boils down to artistic choice.
Having the line-weight vary a bit makes it a bit less cartoony and more soft, ethereal.
It also makes things feel less rigid IMO. Just gotta feel this one out really and see for yourself what the general vibe is you're getting. To really get a feel for it one thing you could do is take one of the screenshots from your original post and draw solid lines over it and see how different the vibe gets in general.

I think the line artist(s) are in charge of this and I don't believe there's much VFX happening on this end tbh.

I personally like this style a lot too!