How does decreasing surface tension of lipids help to emulsify them?
There appears to be either a typo or transcription error in your textbook. 

According to this Nuffield Foundation and Royal Society of Chemistry article 


  Molecules of most detergents and soaps are long chain hydrocarbon
  molecules with an ionic group at one end, usually carrying a negative
  charge, thus making it an anion. This charge is balanced by the
  opposite charge of a soluble cation, for example Na+. The long
  hydrocarbon chains do not interact well with water molecules, and many
  of them are effectively ‘squeezed out’ to the interfaces between the
  water and the air or the glass sides of the beaker. The effect of
  these molecules on the water surface is to considerably weaken the
  forces between water molecules there, thus lowering the surface
  tension.


Soaps, detergents, and bile (all examples of amphiphilic compounds) act to reduce the surface tension of water, not the lipids.