First of all, there is no 45 year rule. I have assessed many grandparents to care for their grandchildren where there was a much larger age gap than 45 years. Part of the assessment is always to look at contingency plans within the family in case the grandparents can't continue to care for the child until adulthood.
Secondly, yes, this is a public law case but the grandparents are presumably not parties in the proceedings. They will not therefore qualify for legal aid.
Thirdly, there may well be other factors here that we are not aware of that have ruled the grandparents out. Alternatively, it could just be a bad decision. It happens. I'm glad it will be heard by a court, thanks to the solicitor who has agreed to represent them free of charge. I will be interested in the outcome, and hope we get to hear it.