I highly doubt its enforceable. I can't try to enforce a ToS against you if I never even asked you to agree to it, or even _informed_ you that such a ToS exist.
EDIT: Did some random googling about enforceability of terms of service. In a case discussed here[1] the ToS were deemed unenforceable based upon in part the lack of assent to the terms.
> [A]cceptance need not be express, but where it is not, there must be evidence that the offeree knew or should have known of the terms and understood that acceptance of the benefit would be construed by the offeror as an agreement to be bound. . .That is not the case here. The plaintiffs were never put on inquiry notice of the arbitration provision, and their continued credit-card payments, which were auto-debited from their credit cards, were too passive for any reasonable fact-finder to conclude that they manifested a subjective understanding of the existence of the arbitration and other emailed provisions and an intent to be bound by them in exchange for the continued benefits [membership] offered.
In the HN case I would argue that there is no evidence that anyone creating a HN account knows or can be expected to know that there are ToS related to that account creation that they are agreeing to, as there is no mention of them in the complete registration process.
[1] - https://www.forbes.com/sites/oliverherzfeld/2013/01/22/are-w...