Care to demonstrate how it's not?
The health expression in Iron Skin's article abides by the abilities UI as well as in-game observations. If you disagree, perform a little test.
Equip a maxed Intensify on your Rhino or Rhino Prime with no other mods and check the abilities UI. Assuming Iron Skin is also maxed, what is the modified health value that you see under "FERRITE ARMOR?" Do you see 1200 x 1.3 + 2.5 x 190 = 2035 for Rhino or 1200 x 1.3 + 2.5 x 275 = 2247.5 for Rhino Prime? Or do you see ( 1200 + 2.5 x 190 ) x 1.3 = 2177.5 for Rhino or ( 1200 + 2.5 x 275 ) x 1.3 = 2453.75 for Rhino Prime?
If it's the latter, then the armor multiplier is affected by power strength.