I apologize the documentation is a bit confusing about the conventions.
Euler angles created from the sl::Pose values as a sl::float3 representing the rotations around the X, Y and Z axes using YZX convention.
By calling getEulerAngles(), you retrieve the values in a float3 in the order float3(X, Y, Z), in the COORDINATE_SYSTEM defined in the InitParameters.
The “YZX convention” is the convention that describes the order in which the rotations are applied, i.e. we first rotate around the Y axis, then Z, then X.
I hope this helps your understanding of the method.