I think it's a mis-conception, brought on by the fact that many of todays tube amps have extended freq response....unlike those built in times past.
The same with solid state. This design, for the most part, has advanced past the grainy midrange, and chalk board high freq's of times past.
In truth...the best of each type do sound somewhat alike. Both have good bass, and good treble....the biggest difference is in the mid-ranges, (lower/mid/upper) midrange.
Some people can't hear these smaller differences (and the reasons for this vary) They need a products of yesteryear, to confirm that there is a difference...or, a new product...with designed-in colorations.
To my ear...tube amps "do not" sound like solid state? But then again...I don't think Krell amps are bright either?.
I think we read things, and then they grow legs.
Dave