Your question was “ ...how Castro could have possibly known that?”
I gave you a possibility. It wasn’t impossible, as you implied.
The Cuban spy (Aspillaga) who defected was manning the radio equipment and searching for CIA activity, towards Miami and Washington DC, (as usual) on the morning of 11/22/63. At approximately 9:00 to 9:30 that morning, he was directed to stop all that and turn his antennas toward Texas and report anything unusual. His reports normally went straight to Castro. He therefore believes Castro knew about LHO’s threat. Aspillaga worked for Cuban intelligence for 35-years before he defected. Everything else he has divulged has checked out as correct. He was also the first one to tell about Castro’s Armageddon letter during the height of the Cuban missile crisis.
I gave you a possibility. It wasn’t impossible, as you implied.I never said or implied that it was impossible. I said that there was nothing feasible about it and that it was unlikely.
The story about Aspillaga doesn't make much sense. Oswald's alleged phone contact, Sylvia Duran was in Mexico, so what significance does it have that Aspillage turned his antennas toward Texas and heard nothing unusual? Also, Aspillage can believe what he wants, but that still does not mean that Castro actually knew about the assassination plan in advance. But if he did, and he heard it from Duran, they would both be part of the conspiracy, simply by not disclosing the information to the authorities.
If you know in advance that a crime is going to be committed and you do nothing, you automatically become complicit in that crime. So, are you now moving away from the LN scenario to a conspiracy?