GeezerVison

Isn't the same problem here with SO_REUSEADDR?

No; the place to look is uvloop/includes/stdlib.pxi

cdef int has_IPV6_V6ONLY = hasattr(socket, 'IPV6_V6ONLY')
cdef int IPV6_V6ONLY = getattr(socket, 'IPV6_V6ONLY', -1)
cdef int has_SO_REUSEPORT = hasattr(socket, 'SO_REUSEPORT')
cdef int SO_REUSEPORT = getattr(socket, 'SO_REUSEPORT', 0)

Note that there's a check to see if the variable exists, and then a python definition of the variable with a default, guaranteeing it's defined. This is how the code handles platforms that don't have the constants defined. For other constants (such as SO_REUSEADDR) there's neither a check nor a python definition of the constant.