I don't know where she gets the idea that serial publishing isn't done currently. Pirateaba, creator of wanderinginn.com has 4342 patreons (10k per month at least). At that point who cares about the NYTimes bestselling list?
Do you happen to know how this author grew her audience? I'm curious.
Being good at writing servicable prose and story. Writing for a genre that has an online following (litrpg isekai in this case). Consistent output (e.g. twice per week -- in this case it is kind of insane: a novella per week). Then post it to sites where the readers for that niche are (e.g. royalroad.com , topwebfiction.com).