Watching the video of the turbine blades spinning like was off balance, that would be the most likely scenario.   If it was a bird strike, there would be a lot more visible damage.  Even though the turbine blades was spinning off balance and causing a wicked vibration, it was still producing thrust so going to assume the lessor of two evils is to leave the faulty engine on as it was producing consistent life giving thrust cause you never know if you shut down the vibrating engine, the "good" engine might have a problem in the time it takes to get back to an airport to land.  At that point the pilot might end up with no engines working and that would be the worst case scenario.  Seems the vibration was unnerving but consistent and not getting worse.  Also turning off the engine might result in the unbalanced fan blade to behave unexpectedly like possibly change vibration harmonics, possibly destroying the bearing and the whole fan blade seizing up. ...