This author has (apparently) a nice tidy small Erdos number. To publish without co-authors denies those co-authors the chance to get (his Erdos number+1).
So clearly he/she is selfish and has the wrong priorities :)
Let's quit beating around the bush: he has the smallest possible Erdős number[1]. If you don't presently have an Erdős number, or wish to improve your Erdős number for bragging rights, co-authoring a paper with him (or any anyone with an equivalent Erdős number) is the best possible outcome. (Obtaining an Erdős number of one is sadly no longer possible.)
> Paul Erdős has an Erdős number of zero.