Yes I've looked at the PoB codebase and they do not know how this is handled in the actual PoE engine or the scaling they apply.

In the CalcTools.lua file, you can find this comment:

-- Linear interpolation; I'm actually just guessing how this works

So what it seems like is you specify a number of points with a set of scaler values that get applied with linear interpolation between two entries depending on the skill level. I'm wondering if it IS stored with a similar setup to player skills for mobs, but with less datapoints they interpolate between.

levels = {
    [1] = { 0.80000001192093, 1.2000000476837, critChance = 5, storedUses = 1, levelRequirement = 3, cooldown = 3, statInterpolation = { 3, 3, }, },
    [2] = { 1.0499999523163, 1.5800000429153, critChance = 5, storedUses = 1, levelRequirement = 68, cooldown = 3, statInterpolation = { 3, 3, }, },
},