Why would malaysia or RR hold back on release of such info- while search was still going on ???
No-one's going to say 'sorry guys, the plane was still flying for hours' when that conflicts with other sources, until they've exhaustively checked their data to verify that it's real. In the meantime, you keep checking the obvious places to make sure it's not there.
However, I wonder whether this really is RR ACARS data, or someone just said 'ACARS' and they assumed it was that data.