Is the increased humidity on modern jets artificial?
A Reuters article confirms the humidity just comes from the passengers, on the 787 at anyway:


  (...) the air is dry and moisture comes mostly from passengers.


And this Flight Global article:


  (...) the system does not add moisture through active humidifiers.


This APEX article has no discussion of higher cabin pressure being an influence on humidity.

Lastly, this Business Insider article says Boeing indicates the higher pressure alleviates the feeling of mountain sickness that can come from being at 8,000 ft vs 6,000 ft.


  [S]ince there isn't a perfect one-to-one correlation between altitude and jet lag, Boeing has taken additional measures to mitigate the symptoms. These measures include an increase in cabin humidity as well as a new air-filtration system.


So, it's real humidity, and not really an effect of cabin pressure.