Art wrote:
"Who says so?"

Many. It is commonly held opinion.

Frederick Emmons Terman writes on page 89 of his 1955 opus:

"When the load impedance is infinite, Eq. (4-14) shows that that the
coeficient of reflection will be 1 on an angle of 0. Under these
conditions the incident and reflected waves will have equal magnitudes
at the load, and the reflection will be such that the voltages of the
incident and reflected waves have the same phase. As a result, the
voltages of the two waves add mathematically so that at the load
E1=E2+EL/2.

Best regards, Richard Harrison, KB5WZI