The first paragraph of this question seems to be predicated on the false assumption that Brexit will not occur without further action ("but given the failure to get the required legislation passed..."), while the article linked to says the opposite: "The current default position - ie, if nothing changes - is for the UK to leave without a deal on 12 April" - which, at least at the time of publication, appears to be correct (and I am not aware of any development that has changed that.)
That article sets out the options available at the time, but is generally free of speculation about which is more likely. One place it does is here: "The European Court of Justice has said the UK could cancel Brexit altogether without the agreement of other nations, but politically, that's not likely to happen." [my emphasis]. If, however, you follow the link from that sentence, you will find that the British government opposed the question even being raised at the ECJ. That is an objective basis for an argument that the British government is unlikely to act on the ruling, whether you like it or not.
More generally, the existence of an option does not, by itself, invalidate arguments that it is not likely to be taken. Similarly, subsequent events do not retroactively invalidate arguments that were reasonable at the time they were made.