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Those two move actions are handled differently:

Inline moderation - XenForo_Model_InlineMod_Thread::moveThreads

Thread tools - XenForo_ControllerPublic_Thread::actionMove

The thread tools option doesn't error when the destination is the same, unlike inline moderation.

I would say the check is more important for inline moderation because completing the action results in the inline selections being cleared.  Compare that with thread tools where nothing is lost in completing the null move, and an error message wastes no less time than opening the thread again or clicking back in your browser.  Nothing is gained by adding a new error message.