There’s a hard core of about 1/3 of the electorate that will vote for the democrat, no matter what the party nominates.
There’s a hard core of about 1/3 of the electorate that will vote for the Republican, no matter what the party nominates.
That poll is claiming that about 1/4 of the mushy middle prefers Mr. Trump to Mr. Biden.
The group of voters preferring Biden is unlikely to shrink; it’s already at rock-bottom. If Mr. Kennedy does not appear on the ballot, who will “his” 16% of voters prefer?
Caveat: Polls are barnyard stuff.