"Technologically mature extraterrestrial intelligences will recognize and use electromagnetic energy, at certain universally known frequencies or bands, as a means to remotely explore the universe, or to detect, signal or communicate with other intelligences."

This assumption may be fundamentally in error. The framework of the sciences as attained by the human race over a fairly short time span no doubt contains enormous lacunae. It may well be that electromagnetic energy is not only flawed as a concept, but is also a crude and ineffective basis for communication as practiced by a "mature extraterrestrial intelligence."

Continuing with this line of speculative thought, it may be that any electromagnetic transmission would be too intrusive, and not in keeping with the advanced ethics of a long-lived civilization that necessarily early on had to move beyond the tribal/racial forms by which we on earth are still entrapped. I imagine that an advanced civilization, knowing that it was concealed from the perceptions of life forms below a certain threshold of development, would not seek to subsume those lifeforms under its knowledges by trumpeting superiority electromagneticlly; this would just mean enslavement of a sort for the inferior lifeforms.

Perhaps if there were a desire for contact and communication issuing from a compassionate motive on the part of the superior civilization, this might take place in the precise subjective experiential framework of the primitive addressee, without infringing on freedom, and in a subtle fashion not explicable by our sciences as developed to date. If all of the above is possible, such communications have in all likelihood already occurred.

An electromagnetically-based transmission between two social orders of approximately earth rank would require that the two societies be coeval (not likely), and contend clumsily with immense expanses of time and space. The two societies wouldn't have that much of useful information to exchange, and would harbor at least latent racial hatreds.