Oscillation of the Drain-Source voltage in flyback converter
why does not this inductance contribute in the frequency of
oscillation of the Drain-Voltage at this moment ?

Fact - The magnetization inductance + the leakage inductance = total primary inductance.
Fact - The magnetization inductance 100% couples to the secondary
Due to the 100% coupling, magnetization inductance transfers 100% of its energy to the secondary leaving only the energy in the primary leakage inductance to be problematic to the primary MOSFET/switch.
In effect, the magnetization inductance is &quot;clamped&quot; by the secondary voltage and, given that in that part of the switching cycle the secondary voltage is fixed, the magnetization inductance voltage also becomes fixed and cannot contribute to primary voltage oscillation. It cannot contribute because in effect, it becomes a voltage source.