A transmission is the device that conveys the power from where it is generated to where it is used. A Constantly Variable Transmission has a continuous range of effective ratios. An electric drive does not. It has a single ratio (or in the case of a bypass clutch, two fixed ratios).
https://en.wikipedia.org/wiki/Continuously_variable_transmission
Trasmission, in this case, is what you find after the petrol engine crankshaft and before the wheel bearings. So both the two electric motors, that can also be generators, are included in the transmission, in this case made by two electric motors, regulator, battery and cables.
In a transmission, by definition the ratio is the number you gain dividing rpms of the power inlet (in cars is petrol or diesel engine) by the outlet (wheels). Manual gearboxes have several ratios, each gear is one, and you can calculate them in two ways: one is counting the number of gear teeth (also the final drive, meaning the pinion and ring gear of the differential), so you will obtain the first gear ratio, the second gear ratio, the third and so on. The other way is dividing the rpms, and also in this case each gear will give you the ratio given by the different rpms.
In our cases, a belt&pulley transmission have an effective continuos range of ratios, because the ratio between inlet and output rpms continuosly changes. So we call it CVT. But also electric drive does the same, because the ratio between inlet and output rpms continuosly changes.
I know, it was the same for me, before studying as mechanical engeneering. Only then I discovered that the continuosly variable transmissions is a big family, called CVT, where belt&pulley are only a sub-branch. Car enthusiasts think that CVT are only the belt&pulley because they only know this kind of cvt, but the family is bigger. The "trick", in the case of I-MMD, is considering the two electric motors not as motors but as part of the whole transmission system, as they are essential to vary the ratio between petrol engine and the wheels. Infact Honda, in the technical papers, calls the I-MMD as E-CVT.
https://www.hondarandd.jp/point.php?pid=966&lang=enHope you can read it, in case I can provide the PDF.