That will preserve the directory structure which isn't what you want. Rclone doesn't have a way of flattening the directory structure at the moment (though it has been requested in the past).
I would use rclone mount and move them around in the shell.
@ncw
Thanks for your answer
Finally I use rclone mount into a network drive on windows10 and search *.mp4 then ctrl+x all to move them from the other directory.
As NCW says - rclone isn't really set up to do file-organizing and sorting. It just has the tools relevant to storage.
It sounds like what you probably would want to do is mount the drive, and then find a good third-party tool that specifically is made to do large-scale reorganization of files. Then just let that act through rclone and do the job that way. While I can't suggest any particular name of such a tool I have seen many examples of them, so do a little googling and I'm sure you will find something relevant you can use.
Or at worst - a little home-scripting would solve this fairly easily, but why reinvent the wheel when there is almost certainly a proper tool for it already?