What aspects of the photoelectric effect can be explained without recourse to the concept of the photon?

According to one source, no quantization is necessary to explain the photoelectric effect!"Einstein proposed that light consisted of quanta, which we call photons. He suggested that the energy in each quantum of light was equal to the frequency multiplied by a constant (Planck's constant) and that a photon with a frequency over a certain threshold would have sufficient energy to eject a single electron, producing the photoelectric effect. It turns out that light does not need to be quantized in order to explain the photoelectric effect, but some textbooks persist in saying that the photoelectric effect demonstrates the particle nature of light." (Quoted from source)