The text says that "in theory at least, triangles of one kind or another can be found photographically in many places". That is at least until you try to find them and they become incredibly elusive!
Triangles, whether physical or implied, give structure to a photograph and therefore the viewer can clearly see what is intended. It is equally true whether the photograph is that of fashion or a family portrait. It could also be a landscape or a still-life.
An exhibition I went to in January of last year had many examples of the use of triangles in some of the most successful or iconic photographs. The first of these is that of Sienna Miller, photographed for American Vogue in Rome, 2006 by Mario Testino. It can be seen here. Sienna Miller in her dress creates the triangle.
In the photograph called Friends of the Spanish Press, 1968, by Malik Sidibe, even though the group portrait has 4 subjects, the structure is such that a triangle is formed. It can be seen here.
The photograph, The Qajar Series, 2001 by Shadi Ghadirian, seen here, makes use of an inverted triangle, joining the faces of the two women to the small table below them which is covered by a white table cloth. As the image is in black and white, the link between the faces and the table is natural.
Don McCullin used triangles in many of his constructs, for example, the photograph of the shell-shocked soldier in Vietnam.