I'd be amazed if this business is what makes them "ultra millionaires" although I'll eat my words if I'm proven wrong.
Firstly, if the merch isn't ready until months after the show airs all the hype will have long ago died down (makes me wonder why Netflix would even want to be connected if the timing doesn't align) - unless she's going to do multiple series, which I just cannot see there being enough interest in.
And secondly, selling enough product to create a significant income stream is going to require enterprise levels of business strategy and management - distribution deals with big retailers, a serious e-commerce platform and marketing strategy, a robust production and fulfilment chain. A basic shopify website and a couple of Netflix pop up stores is not going to deliver that, and H&M have a handful of staff working for them.
They don't even have the basics in place for data management (as evidenced by the email list fiasco), and the haphazard and amateurish pre promotion for the show and the brand just reinforces they haven't a clue what they're doing.