Even if true, there are apparently a bunch of details that still need to be worked out, but honestly: if Marco Rubio is serious about doing everything he can to prevent Donald Trump from becoming the nominee, it’s Ted Cruz or nobody. It’s not even really a stretch for Rubio: him and Cruz are actually fairly close to each other in opinions, and the trash-talk between the two was never going to be particularly problematic in the long term. There’s also the issue of Rubio’s delegates: he doesn’t have control over where they all go, but he has some control over where some of them go, and of course his opinion would carry weight with those of Rubio’s delegates that can freely go elsewhere.
One other note: Marco Rubio has, at this point, gotten one-sixth of the votes cast thus far in the Republican primaries. Contra some of the people quoted in that Politico article, Rubio’s endorsement and support is something that Ted Cruz would rather like to have. Particularly if it comes with some stumping in various critical primary states.