Also, let's not forget Picard is an intensely private person. He showed intense remorse, yes, but only to his brother, who would remain very intimate to him in a sense, despite possible estrangement over the years.
As far as starfleet records are concerned, Picard is a formal prick, gets taken over by the Borg, is liberated, doesn't need give to any apology as he was taken over, and gets to be a formal prick again.
Or at the very least, it may appear that way to S1 Sisko.
(BTW, it would have been very interesting to see how S7 Sisko would have reacted to the same situation).
If anyone has raised those points earlier, my apologies (didn't read through every post of this thread ).