I think its two-fold. There is no law about collusion. The legal issue is conspiring to commit a crime, not colluding to commit a crime. The word "collusion" is thrown around because it describes the general acts of Trump and his campaign, not a specific crime. If Trump or his people were aware that Russia was offering to hack the election or the DNC to help Trump, its conspiracy to commit a crime. And then there is obstruction to cover that up.
I think the "collusion is not a crime" stuff is 1) to create a narrative that working with Russia (or anyone) for dirt on your opponent is simply politics and not criminal. Rudy knows "conspiracy" is absolutely illegal but collusion is not a law so he's sort of saying the correct thing but he knows its just a word game. What he really means is "getting dirt on your opponent isnt a crime". They want the general narrative to be that they might have had meetings, might have accepted dirt, might have suggested to Russia to get that dirt and even timed its release but that's just politics as usual.
I think to what end, Rudy's intent is to create a public sense that sure they did that, but so what? If he came out today and said "Trump conspired with Russia to hack the election to beat Hilary", he's screwed. So they play these games to slowly release that as a general sense of reality. They are, in essence, admitting it while at the same time, marketing their narrative that its no big deal.
And I think the second part might be to draw a line between collusion and conspiracy. They know Mueller can't charge for collusion. So when the report is released, if it has charges, indictments or allegations against Trump, none of them will be "collusion" and Trump can claim victory..."see, there is no collusion", "See, final reports shows NO COLLUSION". And then their defense becomes "sure, we colluded, but we didnt conspire" while at the same time trading on their months of word games to say "this conspiracy charge is nonsense...as we said, collusion isnt a crime and its just politics" and it gives the base and the GOP something to rally around..."look, this is just politics as usual".
They will have made "collusion" so meaningless that by the time it becomes conspiracy, they're controlling the messaging.
It tells me that they absolutely expect to be charged or otherwise accused of conspiracy and obstruction. They are mounting their defense already. They will then say, if there is no collusion, there can be no conspiracy and if there is no conspiracy there can be no obstruction.
Im not legal eagle, but thats my take. And its desperate.