It seems to me to be the right decision to suspend judgement. Complying with the law also includes accepting any extension, which Johnson has yet to do, but as no such extension has been offered he doesn't have much choice in the matter.
All of this may well be rendered irrelevant by the events of the next ten days anyway, and I suspect that is the real reason why the Courts are holding back from any decision. Parliament is likely to vote through the Withdrawal Agreement Bill at Second Reading tomorrow -- because how can they not? -- and, depending on what happens between second and third reading, matters may move fast enough that we either do leave on 31st, or any delay is small enough that Johnson can feel justified to claim that leaving was "done" in theory, if not in practice, by that date, and the extra two weeks or so were merely to allow to be completed everything that was needed to be done in a more or less orderly manner.
Or MPs may decide to reject the current deal on its own merits, and if in that situation Johnson refuses to accept an offered extension -- or if the EU refuses, citing Johnson's second letter -- then the courts would step in, because at that point politics would have ended and breaking the law would have started. However grating it may be, due process of law also requires deference to politics from time to time.