You're right that public dissents from cert. denial are not as common as dissents from SCOTUS opinions. However, given the questions presented, the current composition of the Court, and the fact that only 4 justices are needed to grant cert., it still seems unlikely that the Court would rule in favor of a different defendant in a different case.

That is, this case presented a novel issue, and it was at a stage where the questions were legal in nature (not factual). That's the perfect opportunity for SCOTUS to take the case if they planned to interpret the ADA as not applying to business' websites. They could hypothetically weigh in on the case after Dominos has a full trial with all the facts, but if they planned to say that as a matter of law the ADA does not apply to Dominoes' website, why would they wait to do so?

And since both "sides" (liberal and conservative) of the Court have at least 4 justices right now, either could have independently granted cert. This is especially true for the conservative wing, which is typically viewed (or typically caricatured, depending on your perspective) as being more business-friendly — and which has 5 justices.

Regardless, thanks for raising this point. You're right that the absence of a cert. denial dissent does not strictly indicate a unanimous vote.