correct. I bought a Pen F with the 20Mpx sensor and noticed NO improvement over the 16 Mpx. I keep using the EM1 Mk1, in combination with a Canon FF. And people usually do not notice difference between pictures from the latter two
I see a lot of difference between the 16MP EM-1 I and the 20MP EM-1 II and EM-1 X. Not so much resolution, but colors are better, no need for saturation, and far better handling of difficult dynamic range issues such as shooting a scene with a snowy peak and closer-in vegetation. The EM-1 I sensor could not handle that situation if it was sunny.
That said, the 20MP EM-1 II produces excellent images; I would doubt I would find significant differences with the OM-1 or with FF. At least not differences worth getting excited about.