Some is, some ain't.
1) Yes, the most readily available (in home improvement stores, etc) EPS has a lower density than the most commonly available XPS
2) XPS is stronger than EPS of (almost) equivalent weight:
..........................Density.........compressive strength................flexural strength
EPS Type II: 1.5 lb/cf.................15-21 psi.....................................35 psi
XPS Type IV: 1.55 lb/cf..............25 psi............................................50 psi
Other:
The XPS most commonly used for wing cores is, I believe, 2.2 lb/cf Type VII. It has a compressive strength of 60 PSI and flexural strength of 75 PSI.
EPS is available at densities lower than any XPS. The lightest EPS is 1 lb/cf, has a compressive strength of 10-14 psi and flexural strength of 25 psi.
Maybe Peter's choice was based on cost or availability. Or, maybe an equivalent weight of very light EPS could be carved into a thicker beam, giving more rigidity than a thinner XPS beam.
Having worked with both a little, I don't think I'd want to use EPS for this.