It doubtless seemed clever to challenge prorogation in Scotland; Scots law differs much from English and their chance of victory was higher. However I can think of two audiences in Scotland who may be unimpressed.
(South of the border there is a huge third audience whom I expect to be very unimpressed, but that is another matter.)
Firstly, a Scot does not need to be unusually honest or unusually lacking in nationalism to wonder how exactly to defend the Scottish supreme court’s ruling on the United Kingdom’s parliament. Every Scot can defend Scotland’s supreme court bullying the MSP’s who sit in the 11-times-over-budget building at the end of Edinburgh’s Royal Mile, but I’m not the only Scot who “couldna juist charge his memory” over why the propriety of a Westminster-affecting act is being ruled on by a provincial court. (There will be some justification, of course – but, in the words of “1066 and all that”, it will not be memorable.)
Secondly, her majesty is in Scotland for the summer, as usual. Also as usual, she has prorogued parliament. It is something she’s accustomed to doing every year at roughly this time for roughly this length of time – but which May did not advise her to do last year during a session whose length was unprecedented in three centuries.
Many a PM has noted how their meetings with the Queen, unlike their meetings with their loyal cabinet colleagues, do not leak. I have no idea whether, after Boris performed his constitutional duty to inform and advise her majesty of prorogation, she chose her constitutional duty of ‘warn’ instead of her constitutional duty of ‘advise’ – but it seems at first glance unlikely she ‘warned’ against something whose unprecedented feature was how long it had been since she’d last done it.
I therefore conjecture that her majesty may think the Scottish supreme court’s action ill-advised rather than her own.
As this ruling is mere PR skirmishing before the UK’s supreme court rules (IIUC, it has no actual effect in itself), it seems to me that the remoaners are again paying a high PR price for a stunt that will play well with themselves but with not so many beyond. The court that travestied historical custom to rule for Gina Miller in 2016 is not the strongest oak for leavers to lean against, but it is being said the case has “no hope” in English law. I hope that’s true, but I anyway think that remoaner lawfare victories have a cost they underestimate.