The footer of the Unlicense web page indicates that the entire page (including the text of the license) is licensed under CC0 Public Domain Dedication.
However, if it weren't licensed that way, the legal principle of estoppel in general prevents someone from punishing you for performing an action that they permitted you to do. The Unlicense web site says:
To opt out of the copyright industry's game altogether and set your code free, put your next software project into the public domain using the following (un)licensing statement:
They have explicitly permitted and instructed you to include the Unlicense licensing statement in your project. If the author of the text of the Unlicense later sued you for violating their copyright after you included the text in your project per their own instructions, their suit would not succeed (even absent CC0 permissions) because you are acting within parameters that the author has explicitly permitted.
If you relied on estoppel (instead of CC0), other actions beyond distribution, such as modifying the text, might not be allowed, since the website doesn't appear to contain any permissions for preparing derivatives of the Unlicense. (However, considering the author's attitudes about copyright in general, I would be personally shocked if they tried to sue you for that.)