The first thing we need to understand is that quantum mechanics is a formalism. In other words, it is a mathematical language in terms of which we model the physical world. There is a difference between physics and formalism. The problem with quantum physics is that it is very difficult to visualize the physical things. So we use the mathematical language to give us some idea of what we are talking about.
When it comes to photons, we cannot really visualize them as physical entities. Instead, we describe them in terms of the concepts from the formalism, like wave functions, state vectors, etc. So when somebody says that a photon is not like an electromagnetic field then they refer to the formalism.
As far as the physics is concerned, we only know that the quantized property of photons are relevant in interactions, because interactions are quantized. This property of interactions has consequences for the fields that are produced by them.
If we perform an experiment where we use such an interaction repeatedly to produce single photons and we make measurements on these photons, we'll find that they behave exactly the same as a classical electromagnetic field would behave. Based on this result (and ignoring the concepts from formalism) we can say that a single photon obeys the same dynamics that a classical electromagnetic field would obey. Therefore, from a purely physics perspective (avoiding any misleading issues from the formalism) we can say that a photon is a single excitation of an electromagnetic field.
