One wonders where these "experts" are found. Completely normal in practice along with others such as "so long" or "good day". In fact, it is so normal and casual that it sounds like a completely routine flight up to that point which is what led me initially to think that something catastrophic happened very suddenly immediately thereafter.
Its being analysed based on the alleged timeframe that the words were spoken after the ACARS was logged out. So if a pilot, which one and any sign of duress, or if not a pilot, then who was speaking to ATC.