I'm not sure what you are babbling about. This was on a call AFTER the assassination. The CIA and FBI were aware of Oswald's visit to the embassy. Oswald was asked about it by the FBI. What would be so mysterious about LBJ learning about this concerning event AFTER the assassination from the FBI or CIA and wanting an update? It likely would have been one of the first things told to him about Oswald after he was identified as the suspect. The suspected presidential assassin was known to have visited the Cuban and Russian embassies just months before the act. It could have meant WWIII if the Cubans or Russians were linked to the assassination. Probably the most critical fact from LBJ's perspective at that moment.
On the question of when LBJ learned about Oswald's visit to MC.
From Max Holland's book "The Assassination Tapes": "During the [Saturday] morning briefing with McCone, Johnson learned some information from the Central Intelligence Agency that is not yet public knowledge - namely that six weeks prior to the assassination the CIA station in Mexico City observed Oswald visiting the Soviet embassy in the Mexican capital."
So LBJ learned about the Oswald visit *before* having the later call that Saturday (11/23) with Hoover on the latest news/information.
What's a bit interesting is that apparently Hoover/McCone did not know at that time that Oswald had also gone to the Cuban consulate. LBJ was only told about the Soviet Embassy visit. So that bit of information hadn't gotten to them yet.
Correction:
LBJ Hoover had that crazy letter that Oswald sent to the Soviet Embassy in Washington. He mentions the Cuban consulate visit there. So Hoover knew about it but doesn't mention if in the call with LBJ? Or was that edited out?