Yes, it is true. I’ve seen pictures of the trial being held outdoors.
https://upload.wikimedia.org/wikipedia/commons/thumb/0/0d/Tennessee_v._John_T._Scopes_Trial-_Outdoor_proceedings_on_July_20%2C_1925%2C_showing_William_Jennings_Bryan_and_Clarence_Darrow._(2_of_4_photos)_(2898243103)_crop.jpg/650px-thumbnail.jpg
Thanks.
(With temps up to 100 this week even here in Maryland, I can appreciate it :-)