Measure the voltage swing in both types of amp. It's pretty much identical. In the Marshall, it's got a single triode stage in front of it; in the TW/Komet, it's got two, but it's also got the tone stack, which drops a fair amount of signal. IOW, it's getting hit pretty much the same in both.
Further, if it was done for the reason you give, there would be no reason for it to exist in the Komet Concorde (since it precedes a cathode follower). But it does. And switching it out (I built a clone years ago, in fact you may recall that I supplied you with the schematics for the K60 and Concorde) noticeably reduces the clipping at any given volume.
And all triodes aren't automatically 'clipping' circuits in the same sense - these are designed to clip asymmetrically with relatively little input signal. Biased to the midpoint, you'd have to hit it much harder to get any clipping at all.