I agree that the words are chosen carefully. I also don't think it's alarming.
But I do think it's revealing of the type of evidence they have. At this point I think it's very likely they have evidence that places RA at the scene, but they don't have strong evidence that he was the actual killer.
That's reflected in the DA's choice of language in this press release ("involved in the murders of Libby and Abby" instead of "murdered Libby and Abby") and it's reflected in the actual charges against Allen. Felony murder is exactly the charge you'd expect for someone involved in a murder kidnapping who didn't commit the actual murder, whereas intentional murder & an additional kidnapping charge is what you'd expect for the actual killer in that scenario. And no, felony murder is not a more serious charge than intentional murder. You get the aggravating factor either way as long as a kidnapping happened.
Now, I don't think that necessarily means that Allen wasn't the actual killer. It's all about what the DA can prove, and he may just not feel confident that they prove that Allen committed the murders himself, even though they believe he did. If that's the case, felony murder is 100% the safer charge.
I do think what we're seeing from the prosecution speaks to a certain lack of clarity of exactly how the crime happened though. But that may not end up being legally important. If they have rock solid evidence that RA was there when it happened, it will be hard for him to avoid a conviction IMO.