But why N2O and not something more inert like plain 'ol CO2 or N2 like in your cold brew coffee or Guinness stout? The article doesn't explain anything.

CO2 is acidic when dissolved in water so would cause the cream to curdle. Nitrogen doesn't readily dissolve so while you can push the liquid out, it wouldn't turn to foam when it hit normal pressure.

AS Lars already remarked, CO2 in water produces H2CO3, which is acidic.

NO2 has a sweet taste cf. https://en.wikipedia.org/wiki/Nitrous_oxide