The load resistance in a MOSFET current mirror vs CMOS
We can say that the load seen by M1 is equal to r_o of M2

Realize that that ignores the fact that M1 itself also has an output impedance \$r_o\$.

the fact that M2 is equivalent to only a resistance is somewhat strange

You need to make a distinction between:
DC (where M2 behaves as a current source) and
AC (or &quot;small signal&quot;) where M2 behaves as a resistance of \$r_o\$.
So in a &quot;first order simplification&quot;, the drain of M2 behaves as a DC current source to \$V_{DD}\$ with a parallel resistance of \$r_{o,M2}\$.
So it is not &quot;double standards&quot; but a simplified model of the circuit depending on what behavior we want to look at.