It is psychological warfare, nothing more. The reason being is that voice transmissions (even ssb) on commericial frequencies is just saying “here I am, look at me”, and is incredibly inefficient for all but the most basic communication. There exist a myriad of off-the-shelf technologies that would be orders of magnitude more secure, resistant to detection and/or decryption, and of higher data throughput. Slow narrow-band mfsk or differential psk (like good old psk31) using hard encryption and turbo code forward error correction, frequency hopping and orthogonal coded spread spectrum using low power to keep it below the noise floor, packet bursts on psuedorandom agreed frequencies, the list goes on. Reading numbers on the air is like those “spy cameras” from the cold war that look clumsy and dorky by today’s standards. And there are better frequencies, like VLF for subs, and the mid-HF range for reliable NVIS (near vertical incident skywave) point-to-point communication. It’s all bluster and psyops.
That is why I suspected the transmission was not directed to subs. Why would they transmit to subs on a SW freq?
However, I can still imagine agents receiving instructions via numeric codes on SW freqs. I have a copy of Passport to Worldband Radio 2009 that shows how N. Korean agents receive instructions this way. Also, I have heard numeric codes in plain voice (Spanish) on SW freqs before.
Of course the actual, intended recipient is speculation on my part.