In-store payments refers to physical store locations. Steam is e-commerce. Wero has been ready for this for almost a year now and if the merchant supports it, it works like a charm.

That said, note that Wero is essentially an instant SEPA transaction so you obviously need a bank account. And, which is maybe not so obvious, your bank needs to support it. Essentially most (or all?) Genossenschaftsbanken and Sparkassen in Germany do; private banks not so much as of yet.