They explicitly side-stepped the motive of why Johnson prorogued Parliament. From page 2 of the summary (https://www.supremecourt.uk/cases/docs/uksc-2019-0192-summar...):
> For present purposes, the relevant limit on the power to prorogue is this: that a decision to prorogue (or advise the monarch to prorogue) will be unlawful if the prorogation has the effect of frustrating or preventing, without reasonable justification, the ability of Parliament to carry out its constitutional functions as a legislature and as the body responsible for the supervision of the executive. In judging any justification which might be put forward, the court must of course be sensitive to the responsibilities and experience of the Prime Minister and proceed with appropriate caution. > If the prorogation does have that effect, without reasonable justification, there is no need for the court to consider whether the Prime Minister’s motive or purpose was unlawful.
Also:
> * MPs voted down the current deal 4 times but EU won't change that deal
The EU is happy to change the deal if the UK government proposes something reasonable. But it will not back down from its red lines which is the integrity of the single market and solidarity with Ireland, which is a member state.