Biden has always had trouble expressing himself. He has trouble with some words, and his stumbles are his efforts to get around what he can't get out of his mouth because he stutters.
I've never met Biden, but one of his oldest friends and closest advisors told me some years ago, following another well-publicized gaffe, that he was much more fluent in private conversation than when speaking extemporaneously in front of TV cameras—a phenomenon this person attributed to the concentration Biden needed to summon to control his stutter in a public setting, which forced him to focus his attention on the way his words
sounded rather than the ideas he intended them to convey. My informant may have been trying in part to apologize for his friend, but his explanation had a ring of truth it.
We've had a number of occasionally inarticulate modern presidents: Eisenhower, Reagan, Ford, both Bushes, Trump, and now Biden. Their garbled syntax and puzzling comments apparently were not caused by muddled thinking, except for Reagan, who probably was suffering from dementia toward the end of his second term, and Trump, whose thoughts tended to veer off in bizarre directions (perhaps the result of dementia but also possibly reflecting his psychological issues).
Biden seems finally to have mastered the use of the teleprompter. Or maybe it's just that his advisors have finally persuaded him to stay on-script, which was a frequent problem while he was vice president. It's an acquired skill and it's more difficult than it superficially might seem to be. He reads comfortably and his intonation is a good match for the meaning of his words. Trump never got the hang of it. When he stayed on script, he sounded like a robot, and when he strayed off script, he sounded ... ahh, how to put it charitably ... like some synapses were experiencing electrical interference.
The most articulate presidents in my experience were Nixon, Kennedy, Clinton, and Obama.