Okay.
But it still smells funny.
Both Cruz and Rubio are both potentially vulnerable to the attack right now, and Obama was in the last two elections.
They'll attack Cruz but not Rubio, and they'll do it to him now when they could have done it to Obama then, but didn't.
Why?
I gave you some of my speculation on the vague "why" question, but another reason the challenges are coming earlier is that interested plaintiff's learned from the Obama challenge experience. Courts stated why the courts would not entertain the cases, and some plaintiffs adapted.
As to why Cruz and not Rubio (or Obama), the legal argument against a person born abroad is much easier to make, and rests on thousands of precedents, all of which are uniform on the subject. The Rubio and Obama cases are more difficult, in light of WKA and Plyler.