They've made these statements because of the known fearmongering that is brought out, most prominently by the CDU/CSU. But red-green-red is by far the most compatible three-party-coalition there is and the probability is high that the Left will forego some of their demands regarding foreign politics if it means to keep the CDU/CSU out of the government. Fear of the Left in the government is unfounded, because it's logical that a party with only 5-7% of the votes won't have much say, so the things that they'll get through will be in neutered form and therefore pretty much align with what the SPD and Greens want and most people don't have a problem with what the SPD and Greens want.
The FPD's wishes are largely incompatible with SPD and Greens, but it's a proposed coalition that doesn't instill fear in Germany's people. For that reason it's seen as more likely than red-green-red, but I doubt it's going to work. Much more likely to end up in another Lindner-moment of "It's better not to govern than to govern wrongly" after months of talks.
On the flipside, the desired partner of the CDU/CSU is the FPD, but these two parties don't look like they'll get close to the necessary votes.
The three most likely outcomes of this election are, in order:
That's assuming that the polls are close to accurate.
Well, you are right that the programs of SPD, Greens and Left are pretty compatible with each other, but the statements were made. Maybe that is to avoid fear-mongering (which is happening anyway), but it seems that there is some substance. Also I doubt the Left will easily let go of their pacifist foreign policies, as this would split the party and make them even more irrelevant. But you are right again, that the Left is pretty weak at the moment.
Greens have shown in the past on provincial level, that they are quick to abandon their ecological agenda for a coalition with the CDU, leaving only some token policies to look like they keep their promises. So despite their strong language regarding climate agenda, I can totally see them give in to be part of a coalition. For the SPD we already know this for a long time, that they give up whatever agenda they have, to be part of a coalition. This lead to SPD voting against their own proposed laws, because they were brought to vote by the opposition and SPD voting for a law only to vote against it a few months later (there was an election in between, and SPD was going from government to opposition).
So overall i think red-green-red is quite unlikely, between the distancing statements and the overall weakness of the Left. It will not be enough for red-green, I highly doubt it (but if there are just enough seats, I actually see that as the most likely coalition). If red-green has not enough seats, then I see them going for CDU or FDP.
A reelection is highly unlikely, as the big parties all probably will lose even more votes. Before that happens, I see CDU voting for a SPD-Green minority government. That would actually put the CDU in a comfortable position to oppose everything while red-green isn't able to get anything substantial done.