Many organisations will make statements that something is happening for clearly stated reasons. Many organisations are also being economical with the truth and in reality other reasons may apply which they simply do not wish to publicly acknowledge whether that be for reputational or legal (or both) reasons.Reason is clearly stated as Furtherance of long running pay dispute.
I don't know the reasons for ASLEF's decision to extend the strike. It certainly seems odd that, for the same basic dispute, that they've decided to have five days back to back on LNER and only LNER whilst at the same time there is media reportage that LNER were the only TOC to undertake a consultation before implementing MSLs.
Could just be a co-incidence. Could be something else. Whilst I don't expect LNER to take ASLEF to court it would be fascinating to see what resulted if they did. I honestly do not know which way it would go.