This follow-through advice may work, but kind of strikes me like ensuring you drive your car to the right place by watching the rear-view mirror.
There are simpler, more direct ways to cure a slice (or a hook), and they begin by understanding the only two things (either one or both) that can cause a ball to have slice spin: 1) outside-in swing path through impact, or 2) open club face at impact.
There are simple techniques to develop the swing mechanics to be on path through impact, and if you employ those, you don't need to think about your follow through one bit. Open club face is different matter and has several possible causes or contributing causes, some of which, though not complicated, are usually overlooked or not understood by by golfers - even golf teachers!
To run through the list is beyond the scope of this forum, and to shoot in the dark with a tip without seeing you or running you through a diagnostic process is irresponsible. It's not complicated, just requires that one know the whole story. More info below...
Best regards,
Doug
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