@Adrien399 Airbnb doesn't allow third-party bookings in the sense that you can cancel third-party instant bookings without penalties.
But, if you want, you are allowed to accept it
Third-party bookings are often problematic because
a) the guest is not an Airbnb member and is not familiar with how it works.
b) the guest often don't read the listing's house rules and description
c) the guest didn't choose your listing (another person did) so he may have different expectations
But, in reality, some people are not very familiar with the internet, smartphones, applications... so their family member has to book for them