When the animation slider moved to look and feel a UI was kept within the compositing KCM that was only visible for non Plasma users so they still had a way to access this.
This non-plasma version still wrote to kwinrc. In theory this is fine as kwin implicitly loads local or global values via KConfig's in built mechanisms. But if it was here future changes to the LNF KCM would not be visible.
We've had too many false positive reports from this setting being used intsead of the kdeglobals value. This patch explicitly writes to kdeglobals so that at least if it does occur everything will remain in sync afterwards.