KjellElec means:

Linearity as such is easily defined, see Wiki.
But this statement from the article is a horror and needs to be dealt with in detail or removed:

"Because tubes are significantly more linear than transistors, tube amplifiers do not need as much global negative feedback to achieve acceptable linearity."

Does he compare triodes, tetrodes or pentodes to bipolar, Jfet or Mosfet transistors, and are they  low-power types or not? Does he compare components or practical circuits?
Tube audio power amps must normally use output transformers, whose wild phase excursions at high frequencies forbid hard overall global feedback in order to stay stable. Thus tube power amps are _forced_ to use a limited amount of global feedback.

End of KjellElec meaning