Pirates v. Phillies--Rules Questions

Phillies have runners on first and third–one out.

Ground ball up the middle that hits the second base umpire before going into center field. Dead Ball. Base hit for the batter, the runner advances to second, but they send the runner back to third?

I thought all runners advanced one base?

No, runners only advance one base if they are forced. When an umpire is hit it’s declared a dead ball. During a dead ball there are two rules, a runner cannot be put out and a runner cannot advance. There for the batter is awarded first base since he cannot be put out. The runner at first cannot be put out and cannot occupy first so he’s forced to second. The runner at third is not a risk of being put out so he does not advance.