Equivalent transformer model AN1679/D
We are referring the secondary inductance to the primary to simplify calculations.
For an impedance $$Z = \frac{V}{I}$$.
If any impedance \$Z_s\$ on the secondary is referred to the primary the equation is:
$$Z_p = \frac{V_p/V_s}{I_p/I_s}Z_s = \frac{N_p/N_s}{N_s/N_p}Z_s = \left(\frac{N_p}{N_s}\right)^2Z_s.$$
This can also be checked by calculating a short-circuit test \$(R_\text{load}=0.)\$  The same ratios apply to the inductance \$L\$.