Calling a full-length (90 minutes or more, but certainly 2 hours!) play a "one act" is an unfortunate perversion of the language, even if technically true.
If it's a stand-alone entertainment, then it is full length, but without an intermission.
What can we possibly gain by confusing CONSTITUTION or GARY with ZOO STORY?
The technically minded should consider that HAMLET was written in five acts, but it hasn't been performed with four intermissions in hundreds of years.