Yes. Choice-of-law terms are frequently found in contracts. When interpreting the law, the court with jurisdiction will do its best to refer to and interpret existing law of the state in question to the case.
The linked opinion's discussion of jurisdiction seems to be about whether the case can be heard in any court, not if there's an alternate venue that would be more proper. FWIW, the retailer's web site[0] does not seem to contain a terms of service or anything with a choice-of-law provision. If the argument is that no court has jurisdiction in which the plaintiff can seek redress, that seems equivalent to saying that the law doesn't apply to the defendants.
Shopify already exercises a huge amount of discretion as to which businesses it's willing to provide services for, and how much it charges them. It does not seem unreasonable to me that the company would either a) be willing to answer to California courts, or b) stop selling its services to California retailers.
That’s why the author said that when the court stated that it would be absurd if there were no state that had jurisdiction, the court was being misleading, because there is always at least one state in which a U.S. entity is subject to jurisdiction.