In an application like a guitar amplifier, some kind of effect/distortion pedal, etc the tube circuit is typically overdriven by design to get all of those warm harmonics. A tube that isn't being overdriven is every bit as capable of being a "wire with gain" as any kind of transistor or more modern circuit.
I get easily annoyed though when people start spewing pseudo-scientific BS justifying that deviations are objectively better than no deviations or that there are significant (or "obvious with good ears") differences between amplifiers where there simply are no such differences.
This isn't solely a defense of the position you challenged, more a call-out on your tactic here. You're not exactly representative.
The talk of transistor amplifiers being perfect and having no sound at all is of course nonsense: typically this kind of claim looks at frequency response and outright ignores slew rate, crossover distortion, behavior as the amp exits its passband on either extreme, and so on. There CAN be transistor amplifiers, both simple and incredibly complex, that are so good that they've got effectively no sound at all. It's even possible that it's easier to get there with a transistor amplifier given enough money and resources. But that's not the claim.