This is absolutely not what I understood after multiple readings.
Thread uses IPV6 for its subnet, but does not communicate on the local network (Matter communicates on the Local network).
In the case of Thread (Matter Over Thread), HA simply needs IPV6 to be enabled in the settings to be able to communicate with Thread, but it is not necessary to have IPV6 enabled on its local network.
My local network is still in IPV4 and Thread works, this confirms that I have understood everything I have read about Thread.
A post that confirms this :