That has a drawing of a "conventional" (Honda-like) CVT that made me think the article was all wrong. It's a paragraph or two later that it explains the Toyota PSD, but even then the picture is wrong. It looks like the MG1 and CE are slaved on a single shaft.
That made interesting reading. Doesn't cover all of the operation, but I can fill in the rest... MG1 must be the "starter motor". MG2 supplies regen braking. But I think I understand it now... The oddities are compromises. It all makes sense.
The Honda Charge/Assist displays what I expect it to display. When I floor the gas pedal, the assist goes full, and stays there. Because the IMA has a power peak at 4000 RPM, I would really expect the bar graph to drop off some above 4000 engine RPM, but maybe that's literary license for the masses, who wouldn't want the graph to reduce while demand is full.
In the Escape, flooring it gives near full assist for a little bit, then swings to charge. That confused me, but it is clearer now. That only happens at higher speeds. (I actually went out and drove the Escape to test my new thoughts.) It is because MG2 is tied to the wheels, and has a peak power at some road speed. I might guess that it's 47mph, where the EPA highway test runs ;-) It is above thirty, and less than sixty. At about 10mph, going up a steep hill, flooring it leaves it at full assist, like I would expect, for the duration of my little test run.
The Escape ICE seems to go to about 4,000 RPM under almost any enthusiastic "gas pedal" position. The MG2 speed would change exactly as the road speed changed, with good power up to a road speed that I could calculate if I went back to Graham's page. The MG1 RPM would change inversely as the road speed increased if the engine stayed at 4,000 RPM.
I don't see how it relates to the "combined HP" being less than additive between the MG2 and the ICE. The MG2 maximum would be related to road speed. The ICE could be held at its maximum HP, and the RPM of that has little to do with the RPM of MG2. The MG1 output would be lower as ICE went higher, so there would eventually be some electrical starvation as the batteries depleted, but it seems that you should be able to see maximum MG2 horsepower added to the maximum ICE horsepower, at least for a few seconds, and maybe only at one particular road speed.