There is an ISO which is not necessarily 200 that is the native sensitivity of the silicon from which the sensor is made. That sensitivity depends on the sensor itself, so will vary between cameras, but it is almost always between ISO 100 and 200.

The camera amplifies the signal to get higher sensitivities. It scales down the signal to get lower ones. Amplification leads to more noise, scaling down leads to loss of gradation which I guess is what your photographer meant by bleeding.

Basically the silicon cannot be changed, it's a physical substance that releases electrons when light lands on it. The amount of electrons determines how illuminated that pixel is but if you must simulate a lower sensitivity, all you can do is divide the number of electrons by some factor.

A simplistic example could be: Imagine a camera with ISO 200 native sensitivity. Each photosite can hold a certain amount of electrons which is read using a 12-bit read-out. That means it can output any value from 0 to 4095. To simulate ISO 100, you just read the photosite and divide by two. This gives you numbers of 0-2047, so there is 1-bit loss of nuances.

The process of scaling down the signal also reduces noise which is why cameras often have a Low ISO. On an Olympus mirrorless for example, it is clearly labelled. When you select ISO 100, it says Low 100 Expansion. This also means that there is sometimes a valid reason to use ISO 100 but usually staying at the native sensitivity is best. As a matter, again on an Olympus, selecting the ISO 200 says ISO 200 Recommended.