PIP Mobility for over 65's

My wife, suffering from MS and other conditions,was originally on DLA mobility and at that time was granted permanently for the rest of her life. However she fell eight days short of the PIP datelineof 68 years of age and had to undertake PIP tests in 2015. Following that test she was awarded personal care but denied mobility. An appeal failed . Since that date her mobility has declined badly and her new PIP test has just been completed (2018) with an award for personal care again but no mobility. Their reply states she can walk 50 but no more than 200 metres despite the fact that we ticked the box for only 50 metres. However the reason for my query today is that it also tells us and I quote:
'The law says we cannot award or increase the mobility part of PIP for claimants aged 65 or over. Whilst I accept your ability to walk has worsened I cannot look at your award as this happened after you reached 65 therefore I cannot award you any PIP for help with mobility'.
In view of the fact that my wife was already on permanent mobility at the age of almost 68 ( well beyond their cut off point of 65) then surely this particular rule would not apply in these circumstances and she should qualify?