I'm not up for arguing points on history in a race I never played personally. So I'll just throw what I know. I'll also put an edit on my last comment to see below.

Speaking to a former "the bunker" GW manager, gave me this info. Lizard men where his main race leading to why I picked up Skaven... to smite him.

In an older edition cold blooded (or what it used to be) did lead to some rolling to move. Latter that idea was moved over to the orks and updated (infighting rules). They figured namely on the black orks, the infighting would balance them out a bit and keep them from just roll tide over the battle.

The rule was removed on lizard men as they felt that the rule was to restrictive on a race that was not getting much love, along with the fact you basically made lots of the troops always being charged and never charging (a big key to success in Warhammer) or unable to reach goals with a dice roll of statistics.

This is when it was moved to cold blood. In the tiny text bullshit GW does (white dwarf, rando interviews ect...), they explained why cold blooded was changed in to the better leadership... being that lizards are largely unexcitable and lazy.

Take that with a grain of your favored seasoning, but that's what the dude at GW said (oh god I'm that guy now).

As for the Slan. Originally I had typed up a longer paragraph on other leaders but felt it was to long for a casual posting on a race I had not played. Along with the fact many on the page likely are not here for a rant on lore... so I kept it TLDR. But you are completely right that the Slan are not the only ones who have moved army's.