Carbon steerer tube scratch (inside the tube) - how bad is it?
Old question.  Not sure if anyone is interested, but here is a shot.
Qualifiyer: I am not a bicycle frame Designer or bike repair person; however I have spent 30 years in the aircraft industry telling people how to repair composites.
That picture is a bit misleading, looks like a flash blew out some color &amp; detail.
The tear line &amp; location makes me think this is a fiberglass isolation ply.  If it is, the floppy piece would be white.  Delaminated fiberglass is normally white, bonded is nearly translucent.  The main use for a thin fiberglass isolation ply would be to prevent galvanic corrosion between the carbon &amp; aluminum.  The aluminum will eventually go away via corrosion, again white.  I am assuming the head stem is aluminum.
Also, fiberglass has a 3X modulus of elasticity over carbon.  It's more stretchy.  Probably making it more resistant to the abuse from your expander plug &amp; putting that extra layer between the plug &amp; carbon.
I see no Structural issue here.  a thin coat of room temperature curing resin, paint or even fingernail polish just over the missing fiberglass with a very slight overlap will take care of any potential galvanic corrosion issues.  If not, it will take a long time for the corrosion to do much damage.