I thought they mentioned this feature in the Phantom rework video, but after playing with and against Phantom there doesn't seem to be such a delay at all.
My thought was that he was going to be more like spy from TF2 where he would need to fully decloak before attacking. Now, I get caught off guard by phantom a whole lot more and I seem to pull off more successful ambushes as well. I'm not sure how to feel about this, but it does give Phantom a great boost in strength.
Also, is there any way to detect Phantoms right as they decloak? When I play Phantom there seems to be a sound that goes off when I decloak, but I don't seem to hear it while playing against Phantom.