In noise voltage equivalent circuit, why are the source and load resistances equal?
In noise voltage equivalent circuit, why are the source and load
resistances equal?

It's all about maximum power transfer to the load resistor from the &quot;noise&quot; resistor. When load and source resistance are equal, the load noise &quot;signal&quot; will be maximum in terms of power.

I don't get though what the lesson meant, especially by worst
condition.

It's not a great choice of words but, it does mean precisely what you suggest in your question; it's all about the maximum power transfer theorem.