Why did RA tell DD he was there until 3:30?
The answer, IMO: he's just not smart.
And it shows.
It was over at 3:30.... so as long as he was "gone" by then, who would know?
He didn't know the juveniles and BB and Libby triangulated him.
He didn't know SC would stick the pin in the end time and create the pyramid.
Didn't know about Libby's phone at all. The timestamps, the video, the audio, the movement, the elevation, the lack of movement...
The girls' bodies were found on the 14th... I daresay, in 2017, only RA knew they were dead by 3:30 on 2/13. I think he felt safe -- he even adjusted his time from 1-3 to 1:30-3:30 iirc during his brief conversation with DD. A sprinkling of truth in the lie...
Important to remember (credit where credit is due, all credit to Libby for filming it and Abby for hiding it) that, without the video, potentially the girls could have been abducted from anywhere within the park, parking lots included, and at any time after drop off. Later, even without the video, Libby's phone would still tell a compelling story, steps, elevation, cessation) but RA didn't know that, didn't know LE would know that.
It is my opinion that RA didn't even consider that the girls had a phone or he would have sent it into the creek (not because of a video he didn't know about but to delay discovery of the crime because of it).
So why didn't he lie about what time he was there? He lied about not seeing the girls, so why not lie about the time too? Maybe it's just what he does. Lies a little, truths a little because somehow it works and people believe him. Life hack.
His comment, "it's not me if ..." is really quite illuminating. In how he thinks. In his brand of logic. Splitting hairs. It's me if I can go home. (Yes, I was there; yes, I was dressed like that; yes, I was on the bridge.) But no, it's not me if it has anything to do with those girls I never met.
Never met? He was there at the same time time as the long-haired babysitter, but I'm guessing it's safe to say he never met her either. It feels like a distancing lie to me. I saw them, I didn't meeeeeet them, therefore, when I say I didn't meet them, I'm (technically) not lying. Liar's wordsmithing.
At the time of that parking lot interview, RA knew he had to be truthful enough in order not to draw hot fire.
Calculated. In a way that made sense to him.
And it worked.
JMO