PBUC 3.14

[greymule]

Can anyone clarify the following from PBUC 3.14?

Play: Bases loaded, two out, score tied in bottom of the ninth inning. Batter hits home run out of ball park. Runner on first, thinking home run automatically wins the game, leaves the baseline and heads towards dugout. He is declared out before the runner from third reaches home plate. Other runners continue around the bases and eventually touch home.

Ruling: No runs score; this is a time play. Game continues in the top of the tenth inning with the score tied.

Fine, but why the clause "before the runner from third reaches home plate"? Even if the runner from third crossed home before the runner the runner from first was called out for abandoning effort, wouldn't that runner's out be the third out on a force play and nullify the run anyway? Or is it just the batter who has to touch first (and the runner from third to touch home for the winning run)? Don't all forced runners have to touch the next base?