That would be, compared to some, a comforting hypothesis.
The problem is, it's almost certainly not true.
A couple months after Pope Benedict's abdication, I read he gave a perfectly coherent 45 minute speech to a bunch of academics, speaking -- as is his style--- brilliantly and without notes.
And just one year ago --- the occasion was the fifth anniversary of Pope Francis assuming the papacy --- Benedict refused to write an endorsement of a book on Pope Francis' theology (LINK) and the Vatican Press Office covered itself with disgrace by trying to make it look like Benedict had endorsed it, and were caught at it. Talk about fake news.
So Benedict's mind --- as he approaches his 92nd birthday in just a couple of days ---- seems to be just fine.
I have a much darker theory, but it involves putting on my stylish tinfoil ecclesiastical war-bonnet.