It is commonly percieved to be so, and the classic examples of ex post facto laws fit that description, but the constitution itself places no such criteria on ex post facto - it merely bans them from being enacted. A law that legalizes a previously illegal action after the fact to benefit a friend is accordingly no less "ex post facto" than one that bans a previously legal action after the fact to harm an enemy.
In the law according to GOPcapitalist, perhaps. The rest of us have to limp along with the legal definition of ex post facto, which is what I said earlier.
And since secession was not illegal up through 1861, any subsequent ruling to the contrary is ex post facto, so this point is moot. Seems to be just another attempt at prolonging the battle since N-S's staunchest ally in the debate has already admitted that secession was not prohibited (WhiskeyPapa, Reply No. 164).