Of course it is both. But imagine it is mostly his memory of events.
I get things mixed up in my head after the stroke happened to me. If you ask me to tell you the story about when my grandfather died, and whether it was before or after my Aunt died in a car wreck, and when it happened in relation to my stroke, and what I was doing through this period related to these events, you will get a story full of falsehoods. All of those events happened within two years of each other, and it is a jumbled mess in my head.
Most of my past life is like this. I cannot remember when something happened for the life of me. I couldn't tell you for certain whether a past event happened 3 years ago or 10, with any certainty. My memory is usually short, for instance I think something happened 3 or 4 years ago, when it was really 9 years ago.
I doubt him getting mixed up about driving a bus as opposed to a truck is a memory issue, but it could be. When your memory is that mixed up, it is hard to tell what your brain is remembering as opposed to a fake memory that you got confused about once, and it caused a recurring misremembrance. But that is not as common. The Amtrak train conductor conversation is almost certainly a misremembrance about time. I can also see him getting mixed up about his grandfathers death and the place where it happened.
It is not a disability of formulating basic thoughts though. Difficulties like this are difficult to explain to somebody who has not gone through it. If they would talk to someone who has memory problems from a major event like a stroke,(or a concussion) but still has most of their faculties, it would behoove them. But they really don't want to get into his problems. That would cause them to admit more fully that he might not be capable of handling the responsibilities of the presidency.