Thanks for pointing that out! I didn't play 5th edition at that time so I've only ever had the copy that is written the way I quoted!
If earlier printing's text describing a critical hit on Pg. 196 is the same as the current edition then there would still be solid evidence to Improved critical...
Pg. 194 PHB:
If The D20 roll for an Attack is a 20, the Attack hits regardless of any modifiers or the target’s AC. This is called a critical hit.
Pg. 72 PHB:
Improved Critical
Beginning when you choose this archetype at 3rd Level, your weapon attacks score a critical hit on a roll of 19 or 20...