What counts in terms of signal quality is the sensor surface area because surface area determines the maximum photon count possible.
With total areas of 864 mm squared for 24x36, 370 for APS-C and 225 for m4/3 sensors, the differences are not small.
Of course sensor pitch, and several other factors significantly impact the overall performance of a sensor as a system. So the surface area only affects the upper limit of performance.
It is clear m4/3 sensors perform well and they getting better. Physics does not support the claim that the very best small sensor performance is similar to the performance of the very best larger sensors.
Today, it is certainly true cameras with smaller sensor areas perform at a high level. The m4/3 system's commercial success represents irrefutable evidence large numbers of photographers find the m4/3 smaller sensor area does not compromise their goals. This means people can enjoy the advantages of a compact camera without significant penalty. But it does not mean a larger sensor area with a similar overall efficiency is no longer important. There will always be situations where more signal makes a significant difference in image quality.
Does anyone buy the highest performing m4/3 camera because it's sensor outperforms the highest performing APS-C equipped camera? Convenience is only reason I carry an APS-C mirrorless camera with me everyday instead of a DSLR with 24x36 mm sensor.