It's the combination of still being able to utter words mostly normally (if we forget the ones that don't) but nonsensical/delusional thoughts swirling around in his head that come out through his mouth... it's quite different from Biden's slowing down and generalised loss of speech clarity/speed.
Seeing his words written down is illuminating. I defy anyone to read this and think that he's not some way along the line... he just can't kep those thoughts in order, and stuff just pops out whenever it fancies.
https://singjupost.com/full-transcript-trumps-interview-at-the-chicago-economic-club/
Here's just a fairly random snippet - this is October 2024