A tube creates distortionMost transistors create a lot more. In fact triodes are extremely linear!
What makes distortion in any amp is not so much the devices used, but **how** they are used: the topology. Some tube amps are inherently extremely low distortion (lower than many transistor amps) and others are obviously not.
However, ’distortion’ by itself is not telling enough. Some types of distortion such as lower ordered harmonics are not objectionable to the ear even in fairly large amounts. Other types such as higher ordered harmonics and intermodulations are objection in vanishingly small amounts, some of which can be difficult to measure.
So far the industry does not place a weighting on distortion, viewing it all on a level playing field while the ear does not. But if they did, tubes would be considered overall extremely low distortion.