It's not a 'shortened' sentence at all. The expression "Get it right, then" is some kind of emphatic riposte to someone who has expressed the need (whatever that may be) to get something right.
The point is that "then" is correct, not "than" and that it is an adjunct, a grammatically omissible item in clause structure.
Perhaps Mr. Matthews would like to address HOW we have arrived in the development of our language at the point where - as we both accept - then' is used for emphasis. Why do we say 'then' and not, for instance, 'than'...or 'there'!