Retired runner called out???

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FED rule 8-4-2 (f):

"Any runner is out when he: as a runner or RETIRED RUNNER, fails to execute a legal slide, or does not attempt to avoid the fielder or the play on a force play at any base"

Situation FED game. R1 on 1st with one out. Batter hits a grounder to short who flips to to the second baseman for the force. R1, already out on the force does not slide but actually overruns second base colliding with the second baseman, who is turning to make the throw to first.

I called the batter/runner out (double play) for the obvious interference.

My question is...If there was NO chance at a double play, and the forced runner is already out (retired runner), how can you call him out AGAIN for not avoiding the fielder? Maybe I'm missing the point of this rule????